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Practice › LeetCode › LeetCode 39xx ›

LeetCode 3926 - Count Valid Word Occurrences

LeetCode Problem 3926 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to count occurrences of specific words in a string that is formed by concatenating an array of smaller string "chunks". The twist is that words are defined in a non-standard way: they can include joiner hyphens , which are hyphens that appear between two lowercase English letters. Any other character, including...

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LeetCode 3843 - First Element with Unique Frequency

LeetCode Problem 3843 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Correctness The proposed solution does not answer the question that Exercise 4.3.1.17 asks. The exercise is specific: given that $v_{n-1} \ge \lfloor b/2 \rfloor$ and $u_n = v_{n-1}$, one must show that the quotient digit $q$ in the division step satisfies $q = b-1$ or $q = b-2$. This is a statement about bounds on the first quotient...

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LeetCode 3672 - Sum of Weighted Modes in Subarrays

LeetCode Problem 3672 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window, Counting, Ordered Set Solution Problem Understanding The problem asks us to calculate the sum of weighted modes for all subarrays of length k in a given array nums . Each subarray is contiguous and has exactly k elements. For a subarray, the mode is the element that occurs most frequently. If multiple elements share the same highest frequency,...

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LeetCode 3761 - Minimum Absolute Distance Between Mirror Pairs

LeetCode Problem 3761 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math Solution Correctness The proposed solution does not answer the question that Exercise 4.3.1.17 asks. The exercise is specific: given that $v_{n-1} \ge \lfloor b/2 \rfloor$ and $u_n = v_{n-1}$, one must show that the quotient digit $q$ in the division step satisfies $q = b-1$ or $q = b-2$. This is a statement about bounds on the first quotient...

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LeetCode 3581 - Count Odd Letters from Number

LeetCode Problem 3581 Difficulty: 🟢 Easy Topics: Hash Table, String, Simulation, Counting Solution Solution Algorithm S performs digit-by-digit subtraction in radix $b$ without requiring knowledge of whether $(u_{n-1}\ldots u_0) b \ge (v {n-1}\ldots v_0) b$. It produces digits $w_0, w_1, \ldots, w {n-1}$ and a final borrow $k \in {0,-1}$, where $k=-1$ occurs if and only if $u < v$. Assume Algorithm S is executed without prior ordering of the...

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LeetCode 3934 - Smallest Unique Subarray

LeetCode Problem 3934 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Rolling Hash, Suffix Array, Hash Function Solution Problem Understanding The problem asks us to find the smallest possible length of a contiguous subarray such that there exists at least one subarray of that length that appears exactly once in the entire array. In other words, for a fixed length L , we look at every subarray of size...

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LeetCode 3927 - Minimize Array Sum Using Divisible Replacements

LeetCode Problem 3927 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Greedy, Number Theory Solution Problem Understanding The problem gives an integer array nums and allows an operation that can reduce values under a divisibility condition. Specifically, you may choose two indices a and b such that nums[a] % nums[b] == 0 , and then replace nums[a] with nums[b] . The effect of this operation is that any number in...

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LeetCode 3923 - Minimum Generations to Target Point

LeetCode Problem 3923 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution LeetCode 3923 - Minimum Generations to Target Point Problem Understanding This problem defines a process that repeatedly creates new 3D points from existing points. We begin with the given list points , which forms generation 0 . For every later generation, we consider all distinct points that have been created so far, including points from every previous generation....

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LeetCode 3859 - Count Subarrays With K Distinct Integers

LeetCode Problem 3859 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sliding Window, Counting Solution Problem Understanding This problem asks us to count subarrays of a given integer array nums such that each subarray contains exactly k distinct integers , and each distinct integer in the subarray appears at least m times . In other words, for a subarray to be valid, it must satisfy two constraints: a cardinality constraint (exactly...

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LeetCode 3810 - Minimum Operations to Reach Target Array

LeetCode Problem 3810 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy Solution Problem Understanding We are given two arrays, nums and target , of equal length. The array nums represents the current state, while target represents the desired final state. In one operation, we choose a value x . Then we locate every maximal contiguous segment whose current value is exactly x . Every position inside those segments is simultaneously...

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LeetCode 3785 - Minimum Swaps to Avoid Forbidden Values

LeetCode Problem 3785 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding This problem asks us to modify an array nums so that for every index i , the element at nums[i] is not equal to the corresponding element at forbidden[i] . The only allowed operation is swapping any two distinct elements of nums . Our goal is to determine the minimum number of swaps required to...

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LeetCode 3740 - Minimum Distance Between Three Equal Elements I

LeetCode Problem 3740 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 3740 - Minimum Distance Between Three Equal Elements I Problem Understanding The problem gives us an integer array nums and asks us to find three distinct indices (i, j, k) such that all three positions contain the same value: nums[i] == nums[j] == nums[k] Such a triple is called a good tuple. For every good tuple, we compute...

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LeetCode 3719 - Longest Balanced Subarray I

LeetCode Problem 3719 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Segment Tree, Prefix Sum Solution Problem Understanding The problem asks us to find the longest contiguous subarray of an integer array nums such that the number of distinct even numbers in the subarray is equal to the number of distinct odd numbers . In simpler terms, a subarray is "balanced" if it has an equal count of...

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LeetCode 3713 - Longest Balanced Substring I

LeetCode Problem 3713 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting, Enumeration Solution Problem Understanding The problem is asking us to find the length of the longest substring of a given string s where all distinct characters in that substring appear the same number of times. In other words, for any candidate substring, if you count how many times each unique character appears, all counts must be identical. The input...

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LeetCode 3678 - Smallest Absent Positive Greater Than Average

LeetCode Problem 3678 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution The crash is now purely an input-format mismatch, and it already reveals exactly where the implementation diverges from the actual problem. What the traceback tells us The program fails here: n, m = map(int, input().split()) with: ValueError: not enough values to unpack (expected 2, got 1) So the input line is: 5 4 1 2 3 1 This means:...

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LeetCode 3659 - Partition Array Into K-Distinct Groups

LeetCode Problem 3659 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution LeetCode 3659 - Partition Array Into K-Distinct Groups Problem Understanding We are given an integer array nums and an integer k . The goal is to determine whether all elements of the array can be partitioned into one or more groups such that every group satisfies two conditions: Each group contains exactly k elements. All elements within a...

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LeetCode 3629 - Minimum Jumps to Reach End via Prime Teleportation

LeetCode Problem 3629 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Breadth-First Search, Number Theory Solution Problem Understanding This problem is essentially about navigating an array using two types of moves: adjacent steps and prime-based teleportation . You start at the first index and want to reach the last index in the minimum number of moves. An adjacent step is straightforward: you can move to the next or previous index...

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LeetCode 3623 - Count Number of Trapezoids I

LeetCode Problem 3623 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Geometry Solution Problem Understanding The problem asks us to count the number of horizontal trapezoids that can be formed from a set of points in 2D Cartesian space. A horizontal trapezoid is defined as a convex quadrilateral that has at least one pair of horizontal sides , meaning two sides are parallel to the x-axis. Since two sides are...

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LeetCode 3583 - Count Special Triplets

LeetCode Problem 3583 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding This problem asks us to count the number of special triplets in an integer array nums . A special triplet (i, j, k) must satisfy three conditions: the indices are strictly increasing 0 <= i < j < k < n , nums[i] is exactly double nums[j] , and nums[k] is also exactly double nums[j] ....

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LeetCode 3551 - Minimum Swaps to Sort by Digit Sum

LeetCode Problem 3551 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to sort an array of distinct positive integers not by their natural numeric value, but by the sum of their digits . If two numbers share the same digit sum, the smaller number must come first. After determining this sorted order, we must calculate the minimum number of swaps needed to transform...

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LeetCode 3568 - Minimum Moves to Clean the Classroom

LeetCode Problem 3568 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation, Breadth-First Search, Matrix Solution LeetCode 3568 - Minimum Moves to Clean the Classroom Problem Understanding We are given a grid representing a classroom. A student starts at the cell marked 'S' and must collect every litter cell marked 'L' . Movement is allowed only in the four cardinal directions, and moving one cell costs exactly one unit of...

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LeetCode 3548 - Equal Sum Grid Partition II

LeetCode Problem 3548 Difficulty: 🔴 Hard Topics: Array, Hash Table, Matrix, Enumeration, Prefix Sum Solution Problem Understanding The problem is asking whether a given m x n matrix of positive integers can be split exactly once , either horizontally or vertically, such that the sums of the two resulting sections are equal, or can be made equal by discounting at most one cell . The catch is that if you...

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LeetCode 3531 - Count Covered Buildings

LeetCode Problem 3531 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting Solution Solution A single-precision floating point number in MIX, as defined in Section 4.2.1, consists of a sign byte, an exponent field, and a normalized fraction stored in one word. A double-precision floating point number in format (1) consists of the same sign and exponent structure, but with an eight-byte fraction split across two words, the most significant five...

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LeetCode 3522 - Calculate Score After Performing Instructions

LeetCode Problem 3522 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Simulation Solution Problem Understanding This problem asks you to simulate a linear sequence of instructions that can either modify a running score or move the instruction pointer by a specified offset. You are given two arrays, instructions and values , both of size n . Each index i represents an instruction. If instructions[i] is "add" , you add values[i]...

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LeetCode 3487 - Maximum Unique Subarray Sum After Deletion

LeetCode Problem 3487 Difficulty: 🟢 Easy Topics: Array, Hash Table, Greedy Solution Problem Understanding The problem asks us to find the maximum sum of a subarray from an integer array nums under two key constraints: all elements in the chosen subarray must be unique, and we are allowed to delete any number of elements (except leaving the array empty) before selecting the subarray. In other words, we are looking for...

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LeetCode 3483 - Unique 3-Digit Even Numbers

LeetCode Problem 3483 Difficulty: 🟢 Easy Topics: Array, Hash Table, Recursion, Enumeration Solution Here’s a comprehensive technical guide for LeetCode 3483 following your requested format: Problem Understanding The problem asks us to determine the number of distinct three-digit even numbers that can be formed from an array of digits called digits . Each digit in the array can only be used once per number , but the array may contain...

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LeetCode 3431 - Minimum Unlocked Indices to Sort Nums

LeetCode Problem 3431 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution LeetCode 3431 - Minimum Unlocked Indices to Sort Nums Problem Understanding We are given two arrays of equal length: nums , where every value is either 1 , 2 , or 3 locked , where each value is either 0 or 1 The array is considered sortable if we can repeatedly perform certain adjacent swaps until the array becomes...

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LeetCode 3443 - Maximum Manhattan Distance After K Changes

LeetCode Problem 3443 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Counting Solution Problem Understanding The input is a string s consisting of the four cardinal directions 'N' , 'S' , 'E' , and 'W' . Starting from the origin (0, 0) , we perform the moves one by one in the order given by the string. After every step, we can compute the Manhattan distance from the origin: $|x|+|y|$...

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LeetCode 3434 - Maximum Frequency After Subarray Operation

LeetCode Problem 3434 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming, Greedy, Enumeration, Prefix Sum Solution LeetCode 3434 - Maximum Frequency After Subarray Operation Problem Understanding We are given an array nums and a target value k . We must perform exactly one operation: Choose a contiguous subarray nums[i..j] . Choose an integer x . Add x to every element inside that subarray. After performing this operation, some elements...

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LeetCode 3404 - Count Special Subsequences

LeetCode Problem 3404 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Enumeration Solution LeetCode 3404. Count Special Subsequences Problem Understanding We are given an array nums of positive integers and need to count how many subsequences of length four satisfy a specific multiplicative relationship. A valid subsequence consists of indices (p, q, r, s) such that: p < q < r < s nums[p] * nums[r] == nums[q] * nums[s]...

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LeetCode 3381 - Maximum Subarray Sum With Length Divisible by K

LeetCode Problem 3381 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution LeetCode 3381 - Maximum Subarray Sum With Length Divisible by K Problem Understanding We are given an integer array nums and an integer k . We must find a contiguous subarray whose length is divisible by k , and among all such valid subarrays, return the maximum possible sum. A subarray is a contiguous segment of the...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2933 - High-Access Employees

LeetCode Problem 2933 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution LeetCode 2933 - High-Access Employees Problem Understanding This problem provides a list of employee access records. Each record contains two pieces of information: The employee's name. A timestamp in 24-hour "HHMM" format. All timestamps belong to the same day, so there is no need to handle dates or day transitions. An employee is considered high-access if they...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1787 - Make the XOR of All Segments Equal to Zero

LeetCode Problem 1787 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Bit Manipulation, Counting Solution LeetCode 1787. Make the XOR of All Segments Equal to Zero Problem Understanding We are given an array nums and an integer k . Every contiguous segment of length k must have XOR equal to 0 after we perform some modifications to the array. Our goal is to change as few elements as possible....

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LeetCode 3158 - Find the XOR of Numbers Which Appear Twice

LeetCode Problem 3158 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation Solution Problem Understanding This problem asks us to find all numbers in the array that appear exactly twice and compute the bitwise XOR of those numbers. The input is an integer array nums . The problem guarantees that every value appears either once or twice, never more than twice. Our task is not to count how many duplicate...

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LeetCode 3438 - Find Valid Pair of Adjacent Digits in String

LeetCode Problem 3438 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives us a string s containing only digits from '1' to '9' . We need to find the first adjacent pair of digits that satisfies two conditions. First, the two digits in the pair must be different. A pair like "22" or "55" is never valid. Second, each digit in the pair must appear...

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LeetCode 3890 - Integers With Multiple Sum of Two Cubes

LeetCode Problem 3890 Difficulty: 🟡 Medium Topics: Hash Table, Sorting, Counting, Enumeration Solution Problem Understanding The problem asks us to find all integers x less than or equal to a given integer n such that x can be expressed as the sum of cubes of two positive integers in at least two distinct ways . A “distinct way” means that the pairs (a, b) used to form x are different,...

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LeetCode 3889 - Mirror Frequency Distance

LeetCode Problem 3889 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution LeetCode 3889 - Mirror Frequency Distance Problem Understanding We are given a string s containing only lowercase English letters ( a-z ) and digits ( 0-9 ). Each character has a corresponding mirror character: Letters are mirrored across the alphabet. a ↔ z b ↔ y c ↔ x and so on. Digits are mirrored across the digit...

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LeetCode 3885 - Design Event Manager

LeetCode Problem 3885 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to design an EventManager class that manages a set of events, each identified by a unique eventId and associated with a priority . The manager must support two primary operations: updating the priority of an active event and retrieving the highest-priority active event, with ties broken by...

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LeetCode 3790 - Smallest All-Ones Multiple

LeetCode Problem 3790 Difficulty: 🟡 Medium Topics: Hash Table, Math Solution Problem Understanding This problem asks us to find the smallest positive integer composed entirely of the digit 1 that is divisible by a given integer k . The input k is guaranteed to be between 2 and 100,000. We are asked not for the number itself, but for the number of digits in this number, which simplifies the task...

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LeetCode 3879 - Maximum Distinct Path Sum in a Binary Tree

LeetCode Problem 3879 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem asks us to find the maximum sum of node values along a path in a binary tree such that all values in the path are distinct . The path can start and end at any node, does not need to pass through the root, and must consist of connected nodes. Each...

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LeetCode 3784 - Minimum Deletion Cost to Make All Characters Equal

LeetCode Problem 3784 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Enumeration Solution Problem Understanding The problem asks us to minimize the total cost of deleting characters from a string such that the resulting string contains only one unique character . We are given a string s of length n and an array cost of the same length, where cost[i] is the cost of deleting the character s[i] . The...

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LeetCode 3779 - Minimum Number of Operations to Have Distinct Elements

LeetCode Problem 3779 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem gives us an integer array nums and defines a very specific operation: in one move, we remove the first three elements of the current array. If fewer than three elements remain, we remove everything that is left. We continue performing this operation until one of two conditions becomes true: The array becomes empty. The remaining...

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LeetCode 3873 - Maximum Points Activated with One Addition

LeetCode Problem 3873 Difficulty: 🔴 Hard Topics: Array, Hash Table, Union-Find Solution Problem Understanding The problem is asking us to simulate a chain reaction of activations on a set of 2D points. Each point is defined by its (x, y) coordinates and all points are distinct. When a point is activated, any point sharing the same x coordinate or y coordinate becomes activated as well. This activation propagates recursively: newly...

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LeetCode 3868 - Minimum Cost to Equalize Arrays Using Swaps

LeetCode Problem 3868 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding The problem requires us to make two integer arrays nums1 and nums2 identical by performing swaps. There are two types of swaps: swaps within the same array, which are free, and swaps between arrays at the same index, which cost 1 per operation. The goal is to determine the minimum cost to make the arrays...

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LeetCode 3866 - First Unique Even Element

LeetCode Problem 3866 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find the first even integer in an array that appears exactly once . In other words, we need to traverse the array and identify even numbers, count their occurrences, and return the earliest one that occurs only a single time. If there is no such even number, we return -1 ....

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LeetCode 3773 - Maximum Number of Equal Length Runs

LeetCode Problem 3773 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to analyze a string s consisting of lowercase English letters and identify runs , which are contiguous sequences of identical characters that cannot be extended further. For example, in "aaabaaa" , the runs are "aaa" , "b" , and "aaa" . The goal is to select runs that all have the same...

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LeetCode 3860 - Unique Email Groups

LeetCode Problem 3860 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution LeetCode 3860 - Unique Email Groups Problem Understanding This problem asks us to determine how many distinct email groups exist after applying a specific normalization process to every email address. Each email consists of two parts separated by the '@' character: The local name appears before '@' . The domain name appears after '@' . Before comparing emails,...

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LeetCode 3852 - Smallest Pair With Different Frequencies

LeetCode Problem 3852 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution LeetCode 3852 - Smallest Pair With Different Frequencies Problem Understanding This problem gives us an integer array nums and asks us to find a pair of distinct values [x, y] that satisfies two conditions: x < y The frequency of x in the array is different from the frequency of y . Among all valid pairs, we must...

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LeetCode 3853 - Merge Close Characters

LeetCode Problem 3853 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem asks us to repeatedly merge close, equal characters in a string s . A character pair is considered close if the distance between their indices is at most k . When a merge occurs, the right character is removed and the string is updated immediately. Merges are performed one at a time, always choosing the...

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LeetCode 3851 - Maximum Requests Without Violating the Limit

LeetCode Problem 3851 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sliding Window, Sorting Solution LeetCode 3851 - Maximum Requests Without Violating the Limit Problem Understanding We are given a list of requests, where each request consists of a user ID and a timestamp. Multiple users can appear in the input, and a user may have multiple requests at different times. For a particular user, the rate limit rule states...

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LeetCode 3846 - Total Distance to Type a String Using One Finger

LeetCode Problem 3846 Difficulty: 🟡 Medium Topics: Hash Table, String Solution LeetCode 3846 - Total Distance to Type a String Using One Finger Problem Understanding This problem asks us to calculate the total finger movement required to type a given lowercase string on a special keyboard layout using only one finger. The keyboard is arranged as: q w e r t y u i o p a s d f...

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LeetCode 3842 - Toggle Light Bulbs

LeetCode Problem 3842 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Simulation Solution LeetCode 3842 - Toggle Light Bulbs Problem Understanding The problem gives us an array bulbs , where each value represents a light bulb number between 1 and 100 . Initially, there are exactly 100 light bulbs, and every bulb is turned off. We process the array from left to right. For each value bulbs[i] : If that...

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LeetCode 3839 - Number of Prefix Connected Groups

LeetCode Problem 3839 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution LeetCode 3839: Number of Prefix Connected Groups Problem Understanding The problem gives us an array of strings words and an integer k . Two words are considered prefix-connected if they share exactly the same first k characters and they come from different indices in the array. A connected group is defined as a set of words where...

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LeetCode 3837 - Delayed Count of Equal Elements

LeetCode Problem 3837 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem requires computing a delayed count for each element in an array nums . For an index i , the delayed count is defined as the number of later elements (indices j > i + k ) that are equal to nums[i] . In simpler terms, for each element, we want to count how many...

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LeetCode 3829 - Design Ride Sharing System

LeetCode Problem 3829 Difficulty: 🟡 Medium Topics: Hash Table, Design, Queue, Data Stream Solution Problem Understanding The problem asks us to design a ride sharing system that manages both riders and drivers as they enter the system, and to match them in a first-come, first-served order. The system must track riders who are waiting for rides, drivers who are available, and support canceling rider requests. Inputs include rider and driver...

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LeetCode 3749 - Evaluate Valid Expressions

LeetCode Problem 3749 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Divide and Conquer, Stack Solution Problem Understanding The problem presents a simplified version of evaluating nested mathematical expressions in string format. The input expression is either a single integer literal, which may be negative, or a nested operation of the form op(a,b) where op is one of the four basic arithmetic operations: add , sub , mul , or...

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LeetCode 3741 - Minimum Distance Between Three Equal Elements II

LeetCode Problem 3741 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem asks us to find three distinct indices (i, j, k) such that the values at those positions are identical: nums[i] == nums[j] == nums[k] Among all such valid triples, we must compute the minimum possible distance , where the distance is defined as: abs(i - j) + abs(j - k) + abs(k - i) If...

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LeetCode 3803 - Count Residue Prefixes

LeetCode Problem 3803 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to analyze prefixes of a string s and count how many of them satisfy a specific condition. A prefix is any substring that starts at the first character and extends to some point in the string. For a prefix to be a residue , the number of distinct characters in that prefix must...

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LeetCode 3739 - Count Subarrays With Majority Element II

LeetCode Problem 3739 Difficulty: 🔴 Hard Topics: Array, Hash Table, Divide and Conquer, Segment Tree, Merge Sort, Prefix Sum Solution Here’s a complete, detailed technical solution guide for LeetCode 3739 - Count Subarrays With Majority Element II following your requested format. Problem Understanding The problem asks us to count the number of contiguous subarrays in which a given target element is the majority element . A majority element is one...

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LeetCode 3737 - Count Subarrays With Majority Element I

LeetCode Problem 3737 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Segment Tree, Merge Sort, Counting, Prefix Sum Solution Here is a comprehensive, detailed reference guide for LeetCode 3737 - Count Subarrays With Majority Element I , following your formatting requirements exactly. Problem Understanding The problem provides an integer array nums and a target integer. The goal is to count the number of subarrays in which the target...

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LeetCode 3731 - Find Missing Elements

LeetCode Problem 3731 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding This problem asks us to reconstruct a missing sequence of integers from a partially incomplete array. We are given an integer array nums containing unique values , and we know an important fact: the array originally contained every integer in a continuous range , but some numbers were removed. Even though some values are missing, the...

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LeetCode 3822 - Design Order Management System

LeetCode Problem 3822 Difficulty: 🟡 Medium Topics: Hash Table, Design Solution LeetCode 3822 - Design Order Management System Problem Understanding This problem asks us to implement a small in-memory order management system that supports four operations on trading orders. Each order has three attributes: orderId , a unique identifier orderType , either "buy" or "sell" price , an integer value When an order is added, it becomes active. Orders remain...

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LeetCode 3728 - Stable Subarrays With Equal Boundary and Interior Sum

LeetCode Problem 3728 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to count stable subarrays in an integer array capacity . A subarray capacity[l..r] is stable if it satisfies two conditions: its length is at least 3, and the first and last elements are each equal to the sum of the elements strictly between them. In other words, for a subarray [a0,...

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LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters

LeetCode Problem 3816 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Greedy, Monotonic Stack Solution LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters Problem Understanding We are given a string s consisting only of lowercase English letters. The allowed operation is very specific: if a character currently appears at least twice in the string, we may delete exactly one occurrence of that character. We may repeat this operation...

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LeetCode 3815 - Design Auction System

LeetCode Problem 3815 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution LeetCode 3815 - Design Auction System Problem Understanding This problem asks us to design a real time auction system that supports inserting, updating, removing, and querying bids efficiently. Each bid is uniquely identified by the pair (userId, itemId) . A user may have at most one active bid for a particular item. If addBid...

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LeetCode 3721 - Longest Balanced Subarray II

LeetCode Problem 3721 Difficulty: 🔴 Hard Topics: Array, Hash Table, Divide and Conquer, Segment Tree, Prefix Sum Solution Problem Understanding The problem requires us to find the length of the longest subarray in an integer array nums where the count of distinct even numbers equals the count of distinct odd numbers . A subarray is a contiguous section of the array, and “distinct” means that duplicate values are counted only...

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LeetCode 3720 - Lexicographically Smallest Permutation Greater Than Target

LeetCode Problem 3720 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Counting, Enumeration Solution Before I write the full guide, I want to verify one important detail: LeetCode 3720 appears to be a very recent problem , and the exact intended optimal approach depends on the precise judge behavior and hidden constraints. Please confirm that the problem title and statement are exactly: LeetCode 3720 - Lexicographically Smallest Permutation Greater Than...

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LeetCode 3714 - Longest Balanced Substring II

LeetCode Problem 3714 Difficulty: 🟡 Medium Topics: Hash Table, String, Prefix Sum Solution Problem Understanding The problem gives us a string s that contains only three possible characters: 'a' , 'b' , and 'c' . We must find the longest substring where every distinct character in that substring appears the same number of times . A key detail is that the condition only applies to distinct characters present in the...

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LeetCode 3712 - Sum of Elements With Frequency Divisible by K

LeetCode Problem 3712 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to compute the sum of all elements in the array whose total frequency is divisible by a given integer k . To understand the requirement clearly, we need to focus on two important details: First, we must determine how many times each number appears in the array. This is the element's frequency...

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LeetCode 3811 - Number of Alternating XOR Partitions

LeetCode Problem 3811 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming, Bit Manipulation Solution LeetCode 3811 - Number of Alternating XOR Partitions Problem Understanding We are given an array nums and two distinct target XOR values, target1 and target2 . A partition divides the array into contiguous, non-empty blocks. Every element must belong to exactly one block, and the blocks must cover the entire array. The partition is considered...

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LeetCode 3805 - Count Caesar Cipher Pairs

LeetCode Problem 3805 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, String, Counting Solution LeetCode 3805 - Count Caesar Cipher Pairs Problem Understanding We are given an array words containing n lowercase strings. Every string has the same length m . Two strings are considered similar if we can repeatedly apply the following operation to either string: Shift every character forward by one position in the alphabet. The alphabet wraps...

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LeetCode 3804 - Number of Centered Subarrays

LeetCode Problem 3804 Difficulty: 🟡 Medium Topics: Array, Hash Table, Enumeration Solution Problem Understanding The problem asks us to count the number of centered subarrays in a given integer array nums . A subarray is called centered if the sum of its elements is equal to at least one element within that subarray . In simpler terms, for every contiguous sequence of numbers in nums , we need to check...

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LeetCode 3692 - Majority Frequency Characters

LeetCode Problem 3692 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to analyze the frequency distribution of characters in a string and identify a particular group of characters based on how often they appear. We are given a string s containing only lowercase English letters. For every distinct character, we first count how many times it appears. Characters with the same count belong...

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LeetCode 3795 - Minimum Subarray Length With Distinct Sum At Least K

LeetCode Problem 3795 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution LeetCode 3795: Minimum Subarray Length With Distinct Sum At Least K Problem Understanding This problem asks us to find the shortest contiguous subarray whose distinct-value sum is at least k . The important detail is that we do not sum all elements in the subarray. Instead, each distinct value contributes to the sum exactly once, regardless of...

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LeetCode 3755 - Find Maximum Balanced XOR Subarray Length

LeetCode Problem 3755 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation, Prefix Sum Solution LeetCode 3755 - Find Maximum Balanced XOR Subarray Length Problem Understanding This problem asks us to find the longest contiguous subarray that simultaneously satisfies two independent conditions: The bitwise XOR of all elements in the subarray is equal to 0 . The subarray contains an equal number of even and odd elements. We are given...

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LeetCode 3729 - Count Distinct Subarrays Divisible by K in Sorted Array

LeetCode Problem 3729 Difficulty: 🔴 Hard Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to find the number of distinct subarrays in a sorted array nums such that the sum of elements in the subarray is divisible by a given integer k . A subarray is a contiguous segment of the original array, and two subarrays are considered distinct if their sequences of values differ,...

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LeetCode 3718 - Smallest Missing Multiple of K

LeetCode Problem 3718 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to find the smallest positive multiple of k that is not present in a given integer array nums . In other words, if we consider the infinite sequence of numbers k, 2k, 3k, 4k… , we need to identify the first number in this sequence that does not appear in nums . The...

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LeetCode 3690 - Split and Merge Array Transformation

LeetCode Problem 3690 Difficulty: 🟡 Medium Topics: Array, Hash Table, Breadth-First Search Solution LeetCode 3690 - Split and Merge Array Transformation Problem Understanding The problem gives us two arrays, nums1 and nums2 , of equal length n . We are allowed to repeatedly perform a special operation on nums1 . In a single operation, we choose any contiguous subarray, remove it from its current location, and then insert that entire...

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LeetCode 3679 - Minimum Discards to Balance Inventory

LeetCode Problem 3679 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window, Simulation, Counting Solution LeetCode 3679 - Minimum Discards to Balance Inventory Problem Understanding This problem models a stream of arriving items. On each day, exactly one item arrives, and each item has a type represented by an integer. We are given two parameters: w , the size of the sliding window. m , the maximum number of times...

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LeetCode 3654 - Minimum Sum After Divisible Sum Deletions

LeetCode Problem 3654 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming, Prefix Sum Solution LeetCode 3654 - Minimum Sum After Divisible Sum Deletions Problem Understanding This problem gives us an array nums containing positive integers and an integer k . We may repeatedly remove any contiguous subarray whose sum is divisible by k . After each removal, the remaining elements close the gap, meaning the array becomes shorter and...

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LeetCode 3572 - Maximize Y‑Sum by Picking a Triplet of Distinct X‑Values

LeetCode Problem 3572 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue) Solution LeetCode 3572 - Maximize Y-Sum by Picking a Triplet of Distinct X-Values Problem Understanding The problem gives us two arrays, x and y , both of length n . Each position i represents a pair (x[i], y[i]) . We need to select exactly three distinct indices i , j , and k such that...

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LeetCode 3557 - Find Maximum Number of Non Intersecting Substrings

LeetCode Problem 3557 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Greedy Solution Problem Understanding This problem asks us to identify the maximum number of non-intersecting substrings from a given string word such that each substring is at least four characters long and starts and ends with the same character . In other words, we are scanning the string for patterns where the first and last character match and...

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LeetCode 3556 - Sum of Largest Prime Substrings

LeetCode Problem 3556 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Sorting, Number Theory Solution Problem Understanding The problem requires identifying prime numbers from all possible substrings of a given string of digits and summing the three largest unique primes. In simpler terms, given a string s of length up to 10, we must consider every contiguous sequence of digits, convert it to an integer (ignoring any leading zeros), and...

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LeetCode 3505 - Minimum Operations to Make Elements Within K Subarrays Equal

LeetCode Problem 3505 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Dynamic Programming, Sliding Window, Heap (Priority Queue) Solution Problem Understanding You are given an integer array nums , and you want to create at least k non-overlapping subarrays. Every chosen subarray must have length exactly x , and after performing some modifications, every element inside each chosen subarray must become equal. The only allowed operation is increasing or decreasing...

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LeetCode 3412 - Find Mirror Score of a String

LeetCode Problem 3412 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Simulation Solution LeetCode 3412 - Find Mirror Score of a String Problem Understanding We are given a lowercase English string s . Every letter has a unique "mirror" letter obtained by reversing the alphabet. For example: 'a' ↔ 'z' 'b' ↔ 'y' 'c' ↔ 'x' and so on. We process the string from left to right. For each position...

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LeetCode 3760 - Maximum Substrings With Distinct Start

LeetCode Problem 3760 Difficulty: 🟡 Medium Topics: Hash Table, String Solution LeetCode 3760 - Maximum Substrings With Distinct Start Problem Understanding The problem asks us to split a string into as many non-empty substrings as possible, with one important restriction: the first character of every substring must be different from the first character of every other substring. In other words, if we divide the string into several contiguous pieces, and...

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LeetCode 3715 - Sum of Perfect Square Ancestors

LeetCode Problem 3715 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Tree, Depth-First Search, Counting, Number Theory Solution Problem Understanding This problem asks us to compute a sum across all non-root nodes in a tree based on ancestor relationships and the property of perfect squares. We are given a rooted tree of n nodes, represented as an edge list. Each node has a positive integer value from the nums array....

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LeetCode 3694 - Distinct Points Reachable After Substring Removal

LeetCode Problem 3694 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window, Prefix Sum Solution LeetCode 3694 - Distinct Points Reachable After Substring Removal Problem Understanding We are given a movement string s consisting of the four directions: U increases the y-coordinate by 1. D decreases the y-coordinate by 1. L decreases the x-coordinate by 1. R increases the x-coordinate by 1. Starting from (0, 0) , the entire string...

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LeetCode 3684 - Maximize Sum of At Most K Distinct Elements

LeetCode Problem 3684 Difficulty: 🟢 Easy Topics: Array, Hash Table, Greedy, Sorting Solution Problem Understanding This problem requires selecting at most k distinct elements from a given array of positive integers nums such that their sum is maximized. The result must be returned as a list sorted in strictly descending order . The input nums represents a collection of numbers from which we can pick a subset. Each number can...

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LeetCode 3682 - Minimum Index Sum of Common Elements

LeetCode Problem 3682 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding This problem is asking us to identify the smallest sum of indices from two equal-length arrays, nums1 and nums2 , such that the elements at those indices are equal. Formally, we want a pair (i, j) where nums1[i] == nums2[j] , and we want the minimum value of i + j . If no such pair exists,...

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LeetCode 3668 - Restore Finishing Order

LeetCode Problem 3668 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 3668 - Restore Finishing Order Problem Understanding This problem gives us two arrays: order , which represents the finishing order of all participants in a race. friends , which contains the IDs of our friends. The array order is a permutation of the integers from 1 to n , meaning every participant appears exactly once. The position of...

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LeetCode 3664 - Two-Letter Card Game

LeetCode Problem 3664 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting, Enumeration Solution Problem Understanding Each card consists of exactly two lowercase letters. A card is only relevant if it contains the given letter x somewhere in its two positions. During the game, we repeatedly select two cards that satisfy two conditions: Both cards contain the letter x . The two strings differ in exactly one position. After selecting...

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LeetCode 3662 - Filter Characters by Frequency

LeetCode Problem 3662 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks us to filter a string based on character frequency while preserving the original order of characters. We are given a string s consisting only of lowercase English letters and an integer k . The goal is to construct a new string by keeping only those characters whose total frequency in the entire string...

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LeetCode 3663 - Find The Least Frequent Digit

LeetCode Problem 3663 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to find the least frequent digit in the decimal representation of a given integer n . In other words, we need to count how many times each digit from 0 to 9 appears in the number, and then return the smallest digit that occurs the fewest times. The input is a...

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LeetCode 3641 - Longest Semi-Repeating Subarray

LeetCode Problem 3641 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution LeetCode 3641 - Longest Semi-Repeating Subarray Problem Understanding We are given an integer array nums and an integer k . A subarray is considered semi-repeating if the number of distinct values that appear more than once inside that subarray is at most k . To understand this carefully, consider a frequency map for a subarray: If a...

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LeetCode 3636 - Threshold Majority Queries

LeetCode Problem 3636 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Divide and Conquer, Counting, Prefix Sum Solution Problem Understanding The problem asks us to process multiple queries on a given array nums where each query specifies a subarray defined by [li, ri] and a threshold . For each query, we need to find an element in the subarray that appears at least threshold times. If multiple elements meet...

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LeetCode 3625 - Count Number of Trapezoids II

LeetCode Problem 3625 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Geometry Solution Problem Understanding The problem asks us to count the number of unique trapezoids that can be formed from a set of points on a 2D Cartesian plane. Each point is given as a pair of integers representing its x and y coordinates. A trapezoid is defined as a convex quadrilateral with at least one pair of parallel...

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LeetCode 3606 - Coupon Code Validator

LeetCode Problem 3606 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem is asking us to validate a set of coupons based on three criteria: the coupon code must be a non-empty string containing only alphanumeric characters or underscores, the business line must belong to a set of four predefined categories, and the coupon must be active. We are given three arrays of equal length,...

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LeetCode 3607 - Power Grid Maintenance

LeetCode Problem 3607 Difficulty: 🟡 Medium Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Graph Theory, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem gives an undirected graph of c power stations labeled from 1 to c , connected by bidirectional cables. These connections define connected components, which are referred to as power grids. Importantly, these grids are static, meaning connectivity does not change even if stations...

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LeetCode 3597 - Partition String

LeetCode Problem 3597 Difficulty: 🟡 Medium Topics: Hash Table, String, Trie, Simulation Solution LeetCode 3597 - Partition String Problem Understanding We are given a string s and must partition it into segments according to a very specific process. We start at the current position in the string and begin building a segment character by character. As we extend the segment, we continuously check whether that exact segment has already appeared...

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LeetCode 3591 - Check if Any Element Has Prime Frequency

LeetCode Problem 3591 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting, Number Theory Solution Problem Understanding This problem asks us to determine whether any number in an array appears a prime number of times. In other words, for each unique number in the input array nums , we calculate its frequency-the number of occurrences-and check if that frequency is a prime number. If at least one element has a...

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LeetCode 3588 - Find Maximum Area of a Triangle

LeetCode Problem 3588 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Greedy, Geometry, Enumeration Solution LeetCode 3588 - Find Maximum Area of a Triangle Problem Understanding We are given n distinct points on a 2D Cartesian plane. Each point is represented as [x, y] . We must choose any three points that form a non-degenerate triangle, meaning the area must be strictly greater than zero. Among all valid triangles, we...

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LeetCode 3552 - Grid Teleportation Traversal

LeetCode Problem 3552 Difficulty: 🟡 Medium Topics: Array, Hash Table, Breadth-First Search, Matrix Solution Problem Understanding This problem presents a 2D grid traversal scenario with obstacles and teleportation portals. You are given a matrix of characters representing cells. Empty cells '.' are freely passable, '#' represents obstacles that block movement, and uppercase letters 'A' - 'Z' are teleportation portals. The task is to move from the top-left (0, 0) to...

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LeetCode 3545 - Minimum Deletions for At Most K Distinct Characters

LeetCode Problem 3545 Difficulty: 🟢 Easy Topics: Hash Table, String, Greedy, Sorting, Counting Solution LeetCode 3545 - Minimum Deletions for At Most K Distinct Characters Problem Understanding The problem gives us a string s containing lowercase English letters and an integer k . We are allowed to delete any characters from the string, including multiple occurrences of the same character. After performing the deletions, the resulting string must contain at...

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LeetCode 3542 - Minimum Operations to Convert All Elements to Zero

LeetCode Problem 3542 Difficulty: 🟡 Medium Topics: Array, Hash Table, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem requires us to reduce all elements of a given non-negative integer array nums to zero using the fewest number of operations. Each operation allows selecting a contiguous subarray and setting all occurrences of the minimum non-negative integer in that subarray to zero. In essence, we can remove the "smallest" numbers in...

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LeetCode 3543 - Maximum Weighted K-Edge Path

LeetCode Problem 3543 Difficulty: 🟡 Medium Topics: Hash Table, Dynamic Programming, Graph Theory Solution LeetCode 3543 - Maximum Weighted K-Edge Path Problem Understanding We are given a directed acyclic graph (DAG) with n nodes and weighted directed edges. Each edge is represented as [u, v, w] , meaning there is a directed edge from node u to node v with weight w . The goal is to find a path...

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LeetCode 3541 - Find Most Frequent Vowel and Consonant

LeetCode Problem 3541 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem requires analyzing a string s composed of lowercase English letters and determining the sum of the maximum frequency of vowels and the maximum frequency of consonants. Vowels are defined as 'a' , 'e' , 'i' , 'o' , and 'u' . All other letters are consonants. The frequency of a character is the number...

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LeetCode 3532 - Path Existence Queries in a Graph I

LeetCode Problem 3532 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Union-Find, Graph Theory Solution Path Existence Queries in a Graph I Problem Understanding This problem defines an undirected graph implicitly through a sorted array nums . Each index in nums represents a graph node. Two nodes i and j are connected by an edge whenever: $$|nums[i] - nums[j]| \le maxDiff$$ We are then given many connectivity queries. For...

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LeetCode 3527 - Find the Most Common Response

LeetCode Problem 3527 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution LeetCode 3527 - Find the Most Common Response Problem Understanding This problem gives us a two dimensional array of strings called responses . Each responses[i] represents all survey responses collected on the i th day. A response is simply a string such as "good" , "ok" , or "bad" . The important detail is that duplicate responses...

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LeetCode 3518 - Smallest Palindromic Rearrangement II

LeetCode Problem 3518 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Combinatorics, Counting Solution Problem Understanding We are given a string s that is guaranteed to already be a palindrome. We may rearrange its characters, but the final rearranged string must also be a palindrome. Among all distinct palindromic permutations that can be formed from the characters of s , we must return the k -th lexicographically smallest one. If...

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LeetCode 3510 - Minimum Pair Removal to Sort Array II

LeetCode Problem 3510 Difficulty: 🔴 Hard Topics: Array, Hash Table, Linked List, Heap (Priority Queue), Simulation, Doubly-Linked List, Ordered Set Solution LeetCode 3510: Minimum Pair Removal to Sort Array II Problem Understanding We are given an array nums . The operation is highly constrained: Among all adjacent pairs, find the pair with the minimum sum. If multiple adjacent pairs have the same minimum sum, choose the leftmost one. Replace those...

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LeetCode 3509 - Maximum Product of Subsequences With an Alternating Sum Equal to K

LeetCode Problem 3509 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming Solution Problem Understanding The problem asks us to find a subsequence of a given integer array nums such that the alternating sum of the subsequence equals a target integer k , while maximizing the product of the numbers in that subsequence without exceeding a given limit . The alternating sum is defined as the sum of elements at...

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LeetCode 3508 - Implement Router

LeetCode Problem 3508 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design, Queue, Ordered Set Solution Problem Understanding The problem is asking us to design a Router data structure to manage network packets efficiently under memory constraints. Each packet has three attributes: source , destination , and timestamp . The router has a memory limit , meaning it can store only a fixed number of packets. If a new...

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LeetCode 3507 - Minimum Pair Removal to Sort Array I

LeetCode Problem 3507 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Heap (Priority Queue), Simulation, Doubly-Linked List, Ordered Set Solution LeetCode 3507 - Minimum Pair Removal to Sort Array I Problem Understanding We are given an array nums . At any step, we are not free to choose any adjacent pair. Instead, the operation is completely determined by the current state of the array: Find the adjacent pair whose...

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LeetCode 3493 - Properties Graph

LeetCode Problem 3493 Difficulty: 🟡 Medium Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Graph Theory Solution Problem Understanding The problem asks us to model a 2D array of integers, called properties , as an undirected graph. Each row of the array corresponds to a node in the graph. Two nodes are connected if the number of distinct integers they share is at least k . Our goal is...

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LeetCode 3488 - Closest Equal Element Queries

LeetCode Problem 3488 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search Solution Problem Understanding The problem asks us to process a series of queries on a circular array nums . Each query specifies an index, and for that index, we need to find the minimum circular distance to any other index with the same value. The circular distance means that after the last element, the array wraps back to...

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LeetCode 3486 - Longest Special Path II

LeetCode Problem 3486 Difficulty: 🔴 Hard Topics: Array, Hash Table, Tree, Depth-First Search, Prefix Sum Solution LeetCode 3486 - Longest Special Path II Problem Understanding We are given a rooted tree with n nodes. The tree is rooted at node 0 , and every edge has a positive length. Each node also contains a value stored in the array nums . A path is considered special if it satisfies two...

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LeetCode 3484 - Design Spreadsheet

LeetCode Problem 3484 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Matrix Solution LeetCode 3484 - Design Spreadsheet Problem Understanding This problem asks us to design a very small spreadsheet system that supports three operations on cells and one operation for evaluating simple formulas. The spreadsheet always contains exactly 26 columns, labeled 'A' through 'Z' , and a configurable number of rows. Each cell can store an integer value....

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LeetCode 3481 - Apply Substitutions

LeetCode Problem 3481 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort Solution LeetCode 3481 - Apply Substitutions Problem Understanding This problem gives us a collection of replacement rules and a text containing placeholders. Every placeholder has the format %X% , where X is a single uppercase letter corresponding to one of the replacement keys. The important detail is that replacement values may...

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LeetCode 3471 - Find the Largest Almost Missing Integer

LeetCode Problem 3471 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 3471 - Find the Largest Almost Missing Integer Problem Understanding The problem gives us an integer array nums and an integer k . We consider every contiguous subarray of length k . An integer is called almost missing if it appears in exactly one of those size- k subarrays. Our goal is to find the largest integer that...

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LeetCode 3458 - Select K Disjoint Special Substrings

LeetCode Problem 3458 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Greedy, Sorting Solution Problem Understanding The problem asks us to determine whether we can select k disjoint special substrings from a given string s . A special substring is defined as a substring where every character in it does not appear anywhere else in the string. Additionally, the substring cannot be the entire string itself. The input consists...

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LeetCode 3450 - Maximum Students on a Single Bench

LeetCode Problem 3450 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem provides a list of student-seat assignments, where each entry has the form [student_id, bench_id] . Each pair indicates that a particular student is sitting on a particular bench. The goal is to determine which bench has the largest number of unique students sitting on it and return that count. The important detail is that a...

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LeetCode 3442 - Maximum Difference Between Even and Odd Frequency I

LeetCode Problem 3442 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem requires analyzing the frequency of characters in a given string and computing a specific difference. You are given a string s consisting only of lowercase English letters. For each character in the string, you can count how many times it appears. Some characters will appear an odd number of times , and some will...

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LeetCode 3447 - Assign Elements to Groups with Constraints

LeetCode Problem 3447 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem requires assigning elements from one array, elements , to groups defined by another array, groups , under a divisibility constraint. Specifically, for each group groups[i] , we must find the smallest index j in elements such that groups[i] % elements[j] == 0 . If no such element exists, we assign -1 for that group. Multiple...

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LeetCode 3425 - Longest Special Path

LeetCode Problem 3425 Difficulty: 🔴 Hard Topics: Array, Hash Table, Tree, Depth-First Search, Prefix Sum Solution Problem Understanding This problem provides an undirected tree rooted at node 0 with n nodes and weighted edges, where each node has a value defined in the array nums . A special path is a downward path (from ancestor to descendant) such that all node values along the path are unique . The path...

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LeetCode 3422 - Minimum Operations to Make Subarray Elements Equal

LeetCode Problem 3422 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Sliding Window, Heap (Priority Queue) Solution LeetCode 3422 - Minimum Operations to Make Subarray Elements Equal Problem Understanding We are given an array nums and an integer k . In a single operation, we may increase or decrease any element by exactly 1 . Our goal is not to make the entire array equal. Instead, we only need at...

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LeetCode 3416 - Subsequences with a Unique Middle Mode II

LeetCode Problem 3416 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Combinatorics Solution Problem Understanding We are given an array nums , and we must count how many subsequences of length exactly 5 have a very specific property. For any chosen subsequence $$[a,b,c,d,e]$$ the middle element is c , because it is the third element of a length-5 sequence. The subsequence is valid if: c is a mode of the...

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LeetCode 3408 - Design Task Manager

LeetCode Problem 3408 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution LeetCode 3408. Design Task Manager Problem Understanding This problem asks us to design a data structure that maintains a collection of tasks belonging to different users. Each task has three pieces of information: userId , the user who owns the task taskId , a unique identifier for the task priority , which determines execution...

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LeetCode 3375 - Minimum Operations to Make Array Values Equal to K

LeetCode Problem 3375 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 3375 - Minimum Operations to Make Array Values Equal to K Problem Understanding The problem gives us an integer array nums and a target value k . Our goal is to transform the entire array so that every element becomes exactly equal to k . However, we cannot arbitrarily change values. Each operation follows a very specific rule....

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LeetCode 3335 - Total Characters in String After Transformations I

LeetCode Problem 3335 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Dynamic Programming, Counting Solution Problem Understanding The problem gives us a string s and an integer t . We repeatedly apply a transformation exactly t times. During one transformation: Every character from 'a' to 'y' becomes the next character in the alphabet. The character 'z' becomes the string "ab" . After all transformations are complete, we must return the...

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LeetCode 3224 - Minimum Array Changes to Make Differences Equal

LeetCode Problem 3224 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem gives us an integer array nums of even length n and an integer k . We are allowed to change any element in nums to any integer in the range [0, k] . Our goal is to ensure that for all symmetric pairs (nums[i], nums[n - i - 1]) , the absolute difference...

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LeetCode 3176 - Find the Maximum Length of a Good Subsequence I

LeetCode Problem 3176 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming Solution Problem Understanding We are given an integer array nums and a non-negative integer k . We need to find the maximum possible length of a subsequence such that the number of adjacent unequal pairs is at most k . A subsequence is formed by deleting zero or more elements from the array without changing the order of...

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LeetCode 1782 - Count Pairs Of Nodes

LeetCode Problem 1782 Difficulty: 🔴 Hard Topics: Array, Hash Table, Two Pointers, Binary Search, Graph Theory, Sorting, Counting Solution LeetCode 1782 - Count Pairs Of Nodes Problem Understanding We are given an undirected graph with n nodes and a list of edges. Multiple edges between the same pair of nodes are allowed. For any pair of nodes (a, b) , define incident(a, b) as the number of edges connected to...

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LeetCode 2996 - Smallest Missing Integer Greater Than Sequential Prefix Sum

LeetCode Problem 2996 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to find the smallest missing integer in the array that is greater than or equal to the sum of the longest sequential prefix . A sequential prefix means the beginning part of the array where every number increases by exactly 1 compared to the previous number. Formally, for a prefix nums[0..i] ,...

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LeetCode 2309 - Greatest English Letter in Upper and Lower Case

LeetCode Problem 2309 Difficulty: 🟢 Easy Topics: Hash Table, String, Enumeration Solution Problem Understanding The problem gives us a string s containing only uppercase and lowercase English letters. Our task is to find the greatest English letter that appears in both lowercase and uppercase forms somewhere in the string. The answer must be returned as an uppercase letter. To understand this clearly, consider what "greatest" means in this context. Letters...

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LeetCode 3365 - Rearrange K Substrings to Form Target String

LeetCode Problem 3365 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting Solution Problem Understanding This problem asks whether it is possible to rearrange k contiguous equal-length substrings of a string s to form another string t , given that s and t are anagrams. In simpler terms, we are splitting s into k pieces of the same size, then permuting these pieces in any order to see if we can...

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LeetCode 3143 - Maximum Points Inside the Square

LeetCode Problem 3143 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Binary Search, Sorting Solution Problem Understanding The problem asks us to find the maximum number of points that can be contained in a square centered at the origin (0, 0) such that no two points inside the square share the same tag. The square's edges are parallel to the axes, and points on the edges are considered inside. The...

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LeetCode 1942 - The Number of the Smallest Unoccupied Chair

LeetCode Problem 1942 Difficulty: 🟡 Medium Topics: Array, Hash Table, Heap (Priority Queue) Solution Problem Understanding This problem describes a scenario where a group of friends attends a party with an infinite number of chairs labeled from 0 upwards. Each friend has a specific arrival and leaving time. When a friend arrives, they must occupy the smallest-numbered unoccupied chair . When they leave, that chair becomes free immediately and can...

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LeetCode 3184 - Count Pairs That Form a Complete Day I

LeetCode Problem 3184 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find all pairs of indices (i, j) in an array hours such that i < j and the sum of hours[i] + hours[j] is an exact multiple of 24, which is considered a "complete day." In simpler terms, we are trying to count all unique pairs of hours that add up...

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LeetCode 1930 - Unique Length-3 Palindromic Subsequences

LeetCode Problem 1930 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum Solution Problem Understanding The problem asks us to count unique palindromic subsequences of length 3 in a given string s . A palindromic string reads the same forwards and backwards, and in this case, we only care about strings of exactly three characters. A subsequence is formed by deleting zero or more characters from the original...

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LeetCode 3299 - Sum of Consecutive Subsequences

LeetCode Problem 3299 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming Solution LeetCode 3299 - Sum of Consecutive Subsequences Problem Understanding We are given an array nums , and we must consider all non-empty subsequences of that array. A subsequence preserves the original order of elements, but elements do not need to be contiguous. A subsequence is considered consecutive if every adjacent pair differs by exactly +1 or exactly...

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LeetCode 3331 - Find Subtree Sizes After Changes

LeetCode Problem 3331 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Tree, Depth-First Search Solution Problem Understanding We are given a rooted tree with n nodes numbered from 0 to n - 1 . The tree structure is represented by the parent array, where parent[i] is the parent of node i . Since node 0 is the root, its parent is -1 . Each node also has a character assigned...

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LeetCode 2852 - Sum of Remoteness of All Cells

LeetCode Problem 2852 Difficulty: 🟡 Medium Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Matrix Solution LeetCode 2852 - Sum of Remoteness of All Cells Problem Understanding We are given an n × n grid where each cell is either: A positive integer, representing a valid cell with that value. -1 , representing a blocked cell. Movement is allowed only between non-blocked cells that share an edge, meaning up,...

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LeetCode 3071 - Minimum Operations to Write the Letter Y on a Grid

LeetCode Problem 3071 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix, Counting Solution Problem Understanding The problem asks us to determine the minimum number of operations to convert a given n x n grid into a representation of the letter Y, under specific conditions. The grid contains integers 0, 1, or 2. The cells that belong to the Y consist of two diagonals starting from the top corners to the...

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LeetCode 2215 - Find the Difference of Two Arrays

LeetCode Problem 2215 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 2215, Find the Difference of Two Arrays Problem Understanding This problem asks us to compare two integer arrays and identify the unique values that appear in one array but not the other. We are given two 0-indexed integer arrays, nums1 and nums2 . The goal is to return a list containing two separate lists: The first list should...

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LeetCode 3351 - Sum of Good Subsequences

LeetCode Problem 3351 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming Solution Problem Understanding This problem asks us to compute the sum of all good subsequences in a given integer array nums . A subsequence is any sequence derived from nums by deleting zero or more elements without changing the order of the remaining elements. A subsequence is considered good if the absolute difference between any two consecutive elements...

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LeetCode 2352 - Equal Row and Column Pairs

LeetCode Problem 2352 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix, Simulation Solution Problem Understanding The problem gives us a square matrix grid of size n x n , where every element is an integer. We need to count how many (row, column) pairs are exactly equal. More specifically, for every row ri and every column cj , we compare the sequence of elements in that row with the sequence...

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LeetCode 2955 - Number of Same-End Substrings

LeetCode Problem 2955 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting, Prefix Sum Solution Problem Understanding The problem asks us to compute the number of same-end substrings within specified subranges of a given string s . A substring is same-end if its first and last character are identical. We are given multiple queries in the form [li, ri] , each representing a substring s[li..ri] . For each query, we...

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LeetCode 2353 - Design a Food Rating System

LeetCode Problem 2353 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem is asking us to implement a food rating system that supports dynamic updates to the ratings of individual food items and allows querying for the highest-rated food for a given cuisine. Each food has a unique name, belongs to exactly one cuisine, and has an associated rating. The...

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LeetCode 2248 - Intersection of Multiple Arrays

LeetCode Problem 2248 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem is asking us to find the intersection of multiple arrays. Specifically, given a 2D array nums , where each nums[i] is a non-empty array of distinct positive integers , we want to identify which integers appear in every array in nums . The result should be sorted in ascending order. In simpler terms,...

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LeetCode 1804 - Implement Trie II (Prefix Tree)

LeetCode Problem 1804 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to implement a Trie data structure, also known as a prefix tree , which is designed to store strings efficiently in a way that allows fast lookups and prefix queries. We are required to implement the following operations: insert(word) : Add a word to the trie. countWordsEqualTo(word) : Count how many...

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LeetCode 2284 - Sender With Largest Word Count

LeetCode Problem 2284 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to analyze a chat log containing n messages and identify which sender has written the most words overall. Each message is represented as a string in the messages array, and the corresponding sender of that message is at the same index in the senders array. A word is defined as a...

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LeetCode 2261 - K Divisible Elements Subarrays

LeetCode Problem 2261 Difficulty: 🟡 Medium Topics: Array, Hash Table, Trie, Rolling Hash, Hash Function, Enumeration Solution Problem Understanding This problem requires counting the number of distinct subarrays of a given integer array nums that satisfy a specific constraint: each subarray can contain at most k elements divisible by p . A subarray is a contiguous slice of the array, and "distinct" means either differing in length or differing in...

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LeetCode 3138 - Minimum Length of Anagram Concatenation

LeetCode Problem 3138 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives us a string s that was formed by concatenating several strings together, where every piece is an anagram of the same unknown string t . Our task is to determine the minimum possible length of t . An anagram means the characters are the same, only the order may differ. For example: "abc"...

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LeetCode 2506 - Count Pairs Of Similar Strings

LeetCode Problem 2506 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Bit Manipulation, Counting Solution Problem Understanding The problem gives us an array of strings called words . We must count how many pairs of indices (i, j) satisfy two conditions: i < j words[i] and words[j] are similar Two strings are considered similar if they contain exactly the same set of distinct characters, regardless of order or frequency. For...

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LeetCode 2897 - Apply Operations on Array to Maximize Sum of Squares

LeetCode Problem 2897 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Bit Manipulation Solution Problem Understanding The problem presents an integer array nums and a positive integer k . You can perform a bitwise operation on any two distinct elements of the array any number of times. Specifically, for indices i and j , you update nums[i] to (nums[i] AND nums[j]) and nums[j] to (nums[i] OR nums[j]) . After applying...

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LeetCode 3162 - Find the Number of Good Pairs I

LeetCode Problem 3162 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem gives us two integer arrays, nums1 and nums2 , along with a positive integer k . We need to count how many index pairs (i, j) satisfy the following condition: nums1[i] is divisible by nums2[j] * k In mathematical terms, a pair is considered good if: nums1[i] % (nums2[j] * k) == 0 The task...

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LeetCode 2272 - Substring With Largest Variance

LeetCode Problem 2272 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming, Enumeration Solution Problem Understanding This problem asks us to find the maximum possible variance among all substrings of a given string. The string contains only lowercase English letters, and the variance of a substring is defined as the largest difference between the counts of any two characters that both appear in that substring. The important detail is that...

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LeetCode 2950 - Number of Divisible Substrings

LeetCode Problem 2950 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting, Prefix Sum Solution Problem Understanding The problem asks us to analyze a given string word where each lowercase English letter is mapped to a digit according to a classic phone keypad scheme. A substring of the string is considered divisible if the sum of the mapped digits of its characters is divisible by the length of the substring. The...

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LeetCode 2784 - Check if Array is Good

LeetCode Problem 2784 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem defines a special type of array called a "good" array. A good array must be a permutation of: $$base[n] = [1, 2, 3, ..., n-1, n, n]$$ This means the array must contain: Every integer from 1 to n - 1 exactly once The integer n exactly twice No other numbers Total length equal...

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LeetCode 3104 - Find Longest Self-Contained Substring

LeetCode Problem 3104 Difficulty: 🔴 Hard Topics: Hash Table, String, Sorting Solution Problem Understanding The problem asks us to find the longest self-contained substring in a given string s . A substring is self-contained if it does not share any characters with the rest of the string . More formally, for a substring t of s , every character in t must appear only inside t and not elsewhere in...

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LeetCode 2501 - Longest Square Streak in an Array

LeetCode Problem 2501 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Dynamic Programming, Sorting Solution Problem Understanding The problem asks us to identify the longest square streak in an array of integers nums . A square streak is a subsequence of at least length 2 where, after sorting, every element is the square of the previous element. In other words, if the sorted subsequence is [x1, x2, ..., xk]...

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LeetCode 3020 - Find the Maximum Number of Elements in Subset

LeetCode Problem 3020 Difficulty: 🟡 Medium Topics: Array, Hash Table, Enumeration Solution Problem Understanding The problem gives us an array of positive integers and asks us to choose the largest possible subset that can be rearranged into a very specific symmetric structure. The required structure looks like this: $$[x, x^2, x^4, \dots, x^{k/2}, x^k, x^{k/2}, \dots, x^4, x^2, x]$$ where $k$ is a power of two. The sequence grows by...

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LeetCode 2168 - Unique Substrings With Equal Digit Frequency

LeetCode Problem 2168 Difficulty: 🟡 Medium Topics: Hash Table, String, Rolling Hash, Counting, Hash Function Solution Problem Understanding The problem gives us a string s consisting only of digits from '0' to '9' . We must count how many distinct substrings satisfy a special condition: Every digit that appears in the substring must appear the same number of times. A substring is a contiguous portion of the string. The substring...

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LeetCode 3083 - Existence of a Substring in a String and Its Reverse

LeetCode Problem 3083 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem-Type Check This is a Type B, “Prove that” problem. The statement to prove is: Among all triangles determined by 100 points in general position, at most 70% are acute. The proposed proof attempts to establish a universal upper bound on the number of acute triangles. It does not need to characterize equality cases or extremal configurations. However, the...

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LeetCode 1932 - Merge BSTs to Create Single BST

LeetCode Problem 1932 Difficulty: 🔴 Hard Topics: Array, Hash Table, Tree, Depth-First Search, Binary Search Tree, Binary Tree Solution Problem Understanding This problem asks us to take multiple very small binary search trees (BSTs), each with at most three nodes, and attempt to merge them into a single valid BST. Each tree is represented by its root node in the array trees . The only operation allowed is to attach...

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LeetCode 3395 - Subsequences with a Unique Middle Mode I

LeetCode Problem 3395 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Combinatorics Solution LeetCode 3395 - Subsequences with a Unique Middle Mode I Problem Understanding We are given an integer array nums , and we must count how many subsequences of length 5 satisfy a very specific condition: The subsequence must have exactly 5 elements. The middle element of the subsequence, meaning the third element seq[2] , must be the...

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LeetCode 2402 - Meeting Rooms III

LeetCode Problem 2402 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sorting, Heap (Priority Queue), Simulation Solution Problem Understanding The problem is about scheduling meetings into a fixed number of rooms, where each meeting has a unique start time and a defined duration. We have n rooms numbered from 0 to n - 1 , and we are given a list of meetings represented as [start, end) intervals. The task is...

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LeetCode 2502 - Design Memory Allocator

LeetCode Problem 2502 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Simulation Solution Problem Understanding The problem requires designing a memory allocator that manages a fixed-size memory array. Initially, all memory units are free, and the allocator supports two operations: allocating a block of consecutive free memory units for a given ID ( mID ) and freeing all memory units associated with a given mID . Allocation should always select...

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LeetCode 2459 - Sort Array by Moving Items to Empty Space

LeetCode Problem 2459 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sorting Solution Problem Understanding The array contains every integer from 0 to n - 1 exactly once. The value 0 represents the empty space, while every other number represents an item that should eventually appear in sorted order. The operation is unusual compared to normal array sorting problems. In one move, we may take any item and place it into...

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LeetCode 2434 - Using a Robot to Print the Lexicographically Smallest String

LeetCode Problem 2434 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Greedy Solution Problem Understanding In this problem, we are given a string s and a robot that manages another temporary string t . Initially, t is empty. The robot repeatedly performs one of two allowed operations: First, it may remove the first character from s and append it to the end of t . Second, it may remove the...

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LeetCode 1943 - Describe the Painting

LeetCode Problem 1943 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Prefix Sum Solution Problem Understanding The problem describes a painting laid out on a number line. Each segment of the painting is represented as a half-closed interval [start, end) and is painted with a unique color value. Since multiple segments may overlap, different colors can exist simultaneously over the same region. Instead of returning the actual set of colors...

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LeetCode 3305 - Count of Substrings Containing Every Vowel and K Consonants I

LeetCode Problem 3305 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count all substrings of a given string word that satisfy two conditions simultaneously: first, the substring must contain all five vowels 'a' , 'e' , 'i' , 'o' , and 'u' at least once; second, the substring must contain exactly k consonants. The input is a string of lowercase English...

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LeetCode 2780 - Minimum Index of a Valid Split

LeetCode Problem 2780 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to find a minimum index at which a given integer array nums can be split into two non-empty contiguous subarrays, such that both subarrays share the same dominant element as the original array. A dominant element is one that occurs in more than half of the elements of an array. The input...

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LeetCode 2363 - Merge Similar Items

LeetCode Problem 2363 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Ordered Set Solution Problem Understanding The problem gives us two collections of items, items1 and items2 , where every item is represented as a pair: [value, weight] The value acts like a unique identifier for an item, while weight represents that item's associated weight. Within each individual array, values are guaranteed to be unique, meaning no value appears twice...

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LeetCode 2610 - Convert an Array Into a 2D Array With Conditions

LeetCode Problem 2610 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem asks us to transform a one-dimensional integer array nums into a two-dimensional array (a list of lists) while satisfying three specific conditions. First, every element from nums must appear in the 2D array exactly as many times as it appears in the original array. Second, each row of the 2D array must contain only distinct...

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LeetCode 2965 - Find Missing and Repeated Values

LeetCode Problem 2965 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Matrix Solution LeetCode 2965 - Find Missing and Repeated Values Problem Understanding The problem gives us an n x n matrix called grid . Every value in the matrix should normally contain all integers from 1 to n² exactly once. However, there is one mistake in the matrix: One number appears twice. One number is completely missing. Our task...

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LeetCode 3121 - Count the Number of Special Characters II

LeetCode Problem 3121 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem gives us a string word containing uppercase and lowercase English letters. We must count how many letters are considered "special". A character c is special if two conditions are true: The lowercase version of the letter appears somewhere in the string. The uppercase version of the same letter also appears somewhere in the string. Every...

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LeetCode 2910 - Minimum Number of Groups to Create a Valid Assignment

LeetCode Problem 2910 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy Solution LeetCode 2910 - Minimum Number of Groups to Create a Valid Assignment Problem Understanding This problem gives us an array of integers representing balls. Balls with the same value are indistinguishable for grouping purposes, and every group must contain balls of only one value. The challenge comes from the balancing constraint. If we create multiple groups, the largest...

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LeetCode 2766 - Relocate Marbles

LeetCode Problem 2766 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Simulation Solution Problem Understanding The problem gives us an array nums representing the positions of marbles on a number line. Multiple marbles may exist at the same position, so nums can contain duplicates. We are also given two arrays, moveFrom and moveTo , which describe a sequence of move operations. For each index i , we must move every...

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LeetCode 2283 - Check if Number Has Equal Digit Count and Digit Value

LeetCode Problem 2283 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks us to verify a self-descriptive property of a string of digits. You are given a string num of length n , where each character is a digit between '0' and '9'. The requirement is that for every index i in the string, the digit i must appear exactly num[i] times in the entire...

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LeetCode 2094 - Finding 3-Digit Even Numbers

LeetCode Problem 2094 Difficulty: 🟢 Easy Topics: Array, Hash Table, Recursion, Sorting, Enumeration Solution Problem Understanding The problem gives us an array named digits , where every element is a single decimal digit from 0 to 9 . We must use exactly three elements from this array to build valid three digit integers. The important detail is that each digit can only be used as many times as it appears...

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LeetCode 1893 - Check if All the Integers in a Range Are Covered

LeetCode Problem 1893 Difficulty: 🟢 Easy Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to determine whether every integer within a given inclusive range [left, right] is covered by at least one of the intervals specified in the ranges array. Each element of ranges is itself an array [starti, endi] representing an inclusive interval, meaning the integers starti and endi are part of the interval....

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LeetCode 2932 - Maximum Strong Pair XOR I

LeetCode Problem 2932 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation, Trie, Sliding Window Solution Problem Understanding The problem gives us an integer array nums , and we must choose two numbers x and y from the array such that they form a strong pair . A pair is considered strong if: $$|x - y| \leq \min(x, y)$$ Among all valid strong pairs, we must compute the maximum possible...

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LeetCode 1948 - Delete Duplicate Folders in System

LeetCode Problem 1948 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Trie, Hash Function Solution Problem Understanding The problem gives a hierarchical file system represented as a list of absolute paths, where each path is an array of folder names from the root to a leaf folder. From these paths, we must reconstruct the implicit tree structure of folders, where each node represents a folder and edges represent parent-child relationships....

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LeetCode 1941 - Check if All Characters Have Equal Number of Occurrences

LeetCode Problem 1941 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine whether a given string s is good , meaning that all characters that appear in the string occur the same number of times . In other words, if you count the frequency of each distinct character, those frequencies should be identical for the string to be considered good. The input...

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LeetCode 2408 - Design SQL

LeetCode Problem 2408 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding This problem asks us to design a lightweight in memory database system that supports four core operations on multiple tables: Insert a row into a table Remove a row by id Select a single cell value Export all rows in CSV style format Each table has a fixed number of columns. The constructor receives two...

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LeetCode 2491 - Divide Players Into Teams of Equal Skill

LeetCode Problem 2491 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Sorting Solution Problem Understanding This problem asks us to divide a given list of player skill levels into teams of exactly two players such that every team has the same total skill . The chemistry of a team is defined as the product of the skills of the two players, and we are asked to return the sum...

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LeetCode 2107 - Number of Unique Flavors After Sharing K Candies

LeetCode Problem 2107 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution LeetCode 2107 - Number of Unique Flavors After Sharing K Candies Problem Understanding The problem gives us an integer array candies , where each value represents the flavor of a candy. We must give exactly k consecutive candies to our sister. After removing those k candies, we keep the remaining candies for ourselves. Our goal is to...

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LeetCode 2539 - Count the Number of Good Subsequences

LeetCode Problem 2539 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Combinatorics, Counting Solution

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LeetCode 2813 - Maximum Elegance of a K-Length Subsequence

LeetCode Problem 2813 Difficulty: 🔴 Hard Topics: Array, Hash Table, Stack, Greedy, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem presents a 0-indexed 2D array items of length n , where each element represents an item with two values: a profit and a category. You are asked to select exactly k items to form a subsequence . A subsequence preserves the original order of items but can skip elements....

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LeetCode 3080 - Mark Elements on Array by Performing Queries

LeetCode Problem 3080 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Heap (Priority Queue), Simulation Solution Here’s a fully detailed technical solution guide for LeetCode 3080 following your requested format. Problem Understanding The problem gives a zero-indexed array nums of size n consisting of positive integers and a 2D array queries of size m where each query is [indexi, ki] . All elements of nums are initially unmarked. Each query...

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LeetCode 2975 - Maximum Square Area by Removing Fences From a Field

LeetCode Problem 2975 Difficulty: 🟡 Medium Topics: Array, Hash Table, Enumeration Solution Problem Understanding The field is represented as a large rectangle whose outer boundaries are fixed fences. Inside the field, additional horizontal and vertical fences divide the area into smaller sections. A horizontal fence spans the entire width of the field at a specific row coordinate, and a vertical fence spans the entire height of the field at a...

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LeetCode 3032 - Count Numbers With Unique Digits II

LeetCode Problem 3032 Difficulty: 🟢 Easy Topics: Hash Table, Math, Dynamic Programming Solution Problem Understanding The problem asks us to count how many integers in the inclusive range [a, b] contain only unique digits . A number has unique digits if no digit appears more than once within that number. For example, the number 123 has unique digits because 1 , 2 , and 3 each appear exactly once. However,...

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LeetCode 2275 - Largest Combination With Bitwise AND Greater Than Zero

LeetCode Problem 2275 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation, Counting Solution Problem Understanding The problem asks us to find the largest possible group of numbers from the candidates array such that the bitwise AND of every number in that group is greater than 0 . A bitwise AND operation only keeps bits that are set to 1 in every participating number. For example: 6 = 110 14...

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LeetCode 2405 - Optimal Partition of String

LeetCode Problem 2405 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy Solution Problem Understanding The problem asks us to divide a given string s into the minimum number of substrings such that every substring contains only unique characters . In other words, within a single substring, no character may appear more than once. We are given a string of lowercase English letters and must split it into contiguous pieces. Every...

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LeetCode 2848 - Points That Intersect With Cars

LeetCode Problem 2848 Difficulty: 🟢 Easy Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to determine how many integer points on a number line are covered by at least one car. Each car is represented as a range [starti, endi] of integers, inclusive. For example, a car [3,6] covers the points 3, 4, 5, and 6. The input nums is a 2D array of these...

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LeetCode 2135 - Count Words Obtained After Adding a Letter

LeetCode Problem 2135 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Bit Manipulation, Sorting Solution Problem Understanding The problem requires us to determine how many strings in targetWords can be formed from strings in startWords through a specific transformation. The transformation involves two operations: appending a single letter not already present in the string, and then rearranging all letters arbitrarily. Essentially, we are checking whether, by adding one new character...

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LeetCode 1983 - Widest Pair of Indices With Equal Range Sum

LeetCode Problem 1983 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to find the widest subarray (continuous segment) in two binary arrays nums1 and nums2 such that the sum of elements in that segment is equal for both arrays. Formally, we need to find indices (i, j) with i <= j such that: sum(nums1[i..j]) == sum(nums2[i..j]) and maximize the distance j -...

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LeetCode 3044 - Most Frequent Prime

LeetCode Problem 3044 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Matrix, Counting, Enumeration, Number Theory Solution Problem Understanding The problem asks us to traverse a m x n integer matrix and generate numbers by moving in straight lines along eight possible directions: east, south-east, south, south-west, west, north-west, north, and north-east. Starting from any cell, we can append the digits along a chosen path to form numbers. Importantly, numbers...

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LeetCode 2395 - Find Subarrays With Equal Sum

LeetCode Problem 2395 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to determine whether there exist two contiguous subarrays of length 2 in a given array nums that have the same sum. A subarray of length 2 is simply any consecutive pair of elements in the array. The two subarrays must begin at different indices, which means even if they have identical elements, they...

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LeetCode 2418 - Sort the People

LeetCode Problem 2418 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Sorting Solution LeetCode 2418, Sort the People Problem Understanding This problem gives us two arrays of equal length: names[i] represents the name of a person heights[i] represents the height of that same person The two arrays are aligned by index, meaning the person at index i has both the name names[i] and the height heights[i] . The goal is...

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LeetCode 3238 - Find the Number of Winning Players

LeetCode Problem 3238 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution LeetCode 3238 - Find the Number of Winning Players Problem Understanding The problem gives us n players and a list called pick , where each entry is of the form [xi, yi] . This means player xi picked a ball with color yi . A player wins if they have picked strictly more than their player index number...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2671 - Frequency Tracker

LeetCode Problem 2671 Difficulty: 🟡 Medium Topics: Hash Table, Design Solution Problem Understanding The problem asks us to design a data structure, FrequencyTracker , that can efficiently track the frequency of numbers and answer queries about whether any number exists with a specific frequency. The operations are add , which increases the count of a number; deleteOne , which decreases the count of a number if it exists; and hasFrequency...

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LeetCode 2374 - Node With Highest Edge Score

LeetCode Problem 2374 Difficulty: 🟡 Medium Topics: Hash Table, Graph Theory Solution Problem Understanding The problem asks us to analyze a directed graph where each node has exactly one outgoing edge, represented by the array edges . Each index i represents a node, and edges[i] represents a directed edge from node i to node edges[i] . We are required to calculate the edge score for each node, defined as the...

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LeetCode 2598 - Smallest Missing Non-negative Integer After Operations

LeetCode Problem 2598 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Greedy Solution That is a long, structured technical guide with multiple required sections and two full implementations. To keep the quality high and follow your formatting rules exactly, I will provide it in a single comprehensive response. LeetCode 2598 - Smallest Missing Non-negative Integer After Operations Problem Understanding The problem gives us an integer array nums and an integer...

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LeetCode 2170 - Minimum Operations to Make the Array Alternating

LeetCode Problem 2170 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding The problem requires transforming a given integer array nums into an alternating array with the minimum number of operations. An array is alternating if elements at even indices repeat every two steps ( nums[i] == nums[i-2] for i >= 2 ) and elements at odd indices differ from their previous element ( nums[i] != nums[i-1]...

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LeetCode 1935 - Maximum Number of Words You Can Type

LeetCode Problem 1935 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem is asking us to determine how many words in a given string can be fully typed on a malfunctioning keyboard. The keyboard has some broken letter keys that cannot be used. The input consists of a string text containing words separated by spaces, and a string brokenLetters containing the distinct letters that are broken. The...

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LeetCode 3013 - Divide an Array Into Subarrays With Minimum Cost II

LeetCode Problem 3013 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sliding Window, Heap (Priority Queue) Solution Problem Understanding The problem asks us to divide a given array nums into k contiguous subarrays in a very specific way. Each subarray has a cost defined as its first element. The objective is to minimize the total cost , which is the sum of the first elements of all k subarrays. The challenge...

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LeetCode 2287 - Rearrange Characters to Make Target String

LeetCode Problem 2287 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives us two strings, s and target . We are allowed to take characters from s and rearrange them in any order to form copies of target . Each character in s can only be used once. The goal is to determine the maximum number of complete copies of target that can be constructed...

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LeetCode 2154 - Keep Multiplying Found Values by Two

LeetCode Problem 2154 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Simulation Solution LeetCode 2154 - Keep Multiplying Found Values by Two Problem Understanding This problem asks us to repeatedly search for a value inside an integer array and double that value whenever it is found. We begin with the integer original . If that number exists anywhere in the array nums , we multiply it by two and repeat...

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LeetCode 1915 - Number of Wonderful Substrings

LeetCode Problem 1915 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum Solution LeetCode 1915, Number of Wonderful Substrings Problem Understanding The problem asks us to count how many non-empty substrings of a given string are considered "wonderful". A substring is wonderful if at most one character appears an odd number of times inside that substring. The string only contains characters from 'a' to 'j' , which means...

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LeetCode 3237 - Alt and Tab Simulation

LeetCode Problem 3237 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution Problem Understanding The problem asks us to simulate the behavior of switching between open windows using the classic Alt + Tab operation. We are given an array called windows , which represents the current ordering of open windows. The first element of the array is the window currently on top, and the last element is the window at...

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LeetCode 2549 - Count Distinct Numbers on Board

LeetCode Problem 2549 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Simulation Solution Problem Understanding The problem starts with a single integer n written on a board. Every day, we examine every number currently on the board. For each number x , we look for all integers i such that: 1 <= i <= n x % i == 1 Whenever such an integer i exists, we add it to...

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LeetCode 2048 - Next Greater Numerically Balanced Number

LeetCode Problem 2048 Difficulty: 🟡 Medium Topics: Hash Table, Math, Backtracking, Counting, Enumeration Solution Problem Understanding The problem asks us to find the smallest integer strictly greater than n such that the number is numerically balanced. A number is numerically balanced when every digit that appears in the number appears exactly as many times as its value. For example: 22 is balanced because digit 2 appears exactly 2 times. 1333...

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LeetCode 2404 - Most Frequent Even Element

LeetCode Problem 2404 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to identify the most frequent even number in an integer array nums . If multiple even numbers share the highest frequency, we should return the smallest among them. If the array contains no even numbers, the function should return -1 . The input array nums can have up to 2000 elements, each...

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LeetCode 3063 - Linked List Frequency

LeetCode Problem 3063 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Counting Solution Problem Understanding This problem asks us to analyze a singly-linked list of integers and produce a new linked list containing the frequencies of each distinct element in the input list. Specifically, given the head of a linked list containing k distinct elements, we are to return another linked list of length k where each node represents the...

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LeetCode 3311 - Construct 2D Grid Matching Graph Layout

LeetCode Problem 3311 Difficulty: 🔴 Hard Topics: Array, Hash Table, Graph Theory, Matrix Solution Problem Understanding The problem gives us an undirected graph with n nodes labeled from 0 to n - 1 . We must place every node into a 2D grid so that adjacency in the grid matches adjacency in the graph exactly. Two cells are considered adjacent only if they share a side, meaning horizontal or vertical...

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LeetCode 3378 - Count Connected Components in LCM Graph

LeetCode Problem 3378 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Union-Find, Number Theory Solution Problem Understanding We are given an array nums , where each element represents the value assigned to a node in a graph. The graph contains exactly n nodes, one node for each array element. Two nodes are connected by an undirected edge if the least common multiple of their values is less than or equal...

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LeetCode 2514 - Count Anagrams

LeetCode Problem 2514 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Combinatorics, Counting Solution Problem Understanding The problem asks us to count how many distinct anagram sentences can be formed from a given string s . The input string contains one or more words separated by single spaces. An anagram sentence must preserve the structure of the original sentence. Specifically, the i-th word in the new sentence must be a...

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LeetCode 2183 - Count Array Pairs Divisible by K

LeetCode Problem 2183 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Counting, Number Theory Solution Problem Understanding The problem gives us an integer array nums and an integer k . We must count how many index pairs (i, j) satisfy two conditions: i < j nums[i] * nums[j] is divisible by k In other words, for every pair of distinct elements in the array, we check whether their product contains...

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LeetCode 1852 - Distinct Numbers in Each Subarray

LeetCode Problem 1852 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution LeetCode 1852 - Distinct Numbers in Each Subarray Problem Understanding The problem gives us an integer array nums and an integer k . For every contiguous subarray of length k , we must determine how many unique values appear inside that window. A subarray is a continuous portion of the array. Since the window size is fixed...

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LeetCode 3306 - Count of Substrings Containing Every Vowel and K Consonants II

LeetCode Problem 3306 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of a given string satisfy two conditions simultaneously: The substring contains all five vowels, 'a' , 'e' , 'i' , 'o' , and 'u' , at least once. The substring contains exactly k consonants. The input consists of: A lowercase English string word A non-negative integer...

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LeetCode 1980 - Find Unique Binary String

LeetCode Problem 1980 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Backtracking Solution Problem Understanding The problem is asking us to find a binary string of length n that does not exist in a given list nums of unique binary strings, where each string in nums also has length n . In simpler terms, we are given n strings that consist of 0s and 1s, and we need to generate...

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LeetCode 2006 - Count Number of Pairs With Absolute Difference K

LeetCode Problem 2006 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem gives us an integer array nums and an integer k . We need to count how many pairs of indices (i, j) satisfy two conditions: i < j |nums[i] - nums[j]| == k The notation |x| represents the absolute value of x , meaning the distance between two numbers regardless of direction. For example,...

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LeetCode 2953 - Count Complete Substrings

LeetCode Problem 2953 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of a given string are considered "complete". A substring is complete if it satisfies two conditions simultaneously: Every distinct character in the substring appears exactly k times. For every pair of adjacent characters, the absolute difference between their alphabetical positions is at most 2 . The...

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LeetCode 2570 - Merge Two 2D Arrays by Summing Values

LeetCode Problem 2570 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers Solution Problem Understanding The problem gives us two sorted 2D integer arrays, nums1 and nums2 , where every element is a pair of integers in the form [id, value] . Each id uniquely identifies an entry inside its own array, and the corresponding value represents that id's associated number. Since both arrays are already sorted in ascending order...

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LeetCode 2661 - First Completely Painted Row or Column

LeetCode Problem 2661 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution Problem Understanding The problem is asking us to simulate a painting process on a 2D matrix. We are given a 1D array arr of integers and an m x n matrix mat , both containing all integers from 1 to m * n exactly once. For each integer in arr (in order), we "paint" the cell in mat...

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LeetCode 2399 - Check Distances Between Same Letters

LeetCode Problem 2399 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem asks us to verify whether a string follows a very specific spacing rule between repeated characters. We are given a string s that contains only lowercase English letters. Every character that appears in the string appears exactly twice. We are also given an integer array distance of size 26 , where each index corresponds...

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LeetCode 3026 - Maximum Good Subarray Sum

LeetCode Problem 3026 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to find the maximum possible sum of a contiguous subarray where the absolute difference between the first and last element of that subarray is exactly k . More formally, for a subarray nums[i..j] , it is considered good if: $|nums[i]-nums[j]|=k$ We must examine all valid subarrays and return the largest subarray...

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LeetCode 2244 - Minimum Rounds to Complete All Tasks

LeetCode Problem 2244 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Here is a complete, detailed technical solution guide for LeetCode 2244 - Minimum Rounds to Complete All Tasks following your formatting and content requirements. Problem Understanding The problem provides a 0-indexed array tasks , where each element represents a task's difficulty level. Each round, you are allowed to complete exactly 2 or 3 tasks of the same...

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LeetCode 2547 - Minimum Cost to Split an Array

LeetCode Problem 2547 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Counting Solution Problem Understanding The problem asks us to split the array nums into one or more contiguous non-empty subarrays such that the total cost is minimized. For every subarray, we define a special quantity called its importance value. To compute it, we first construct the trimmed version of the subarray by removing every element that appears exactly...

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LeetCode 2935 - Maximum Strong Pair XOR II

LeetCode Problem 2935 Difficulty: 🔴 Hard Topics: Array, Hash Table, Bit Manipulation, Trie, Sliding Window Solution Problem Understanding The problem asks us to find the maximum XOR value of a strong pair in a given array of integers nums . A pair (x, y) is considered strong if it satisfies the condition |x - y| <= min(x, y) . This means the difference between the two numbers cannot exceed the...

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LeetCode 2841 - Maximum Sum of Almost Unique Subarray

LeetCode Problem 2841 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution LeetCode 2841 - Maximum Sum of Almost Unique Subarray Problem Understanding The problem gives us an integer array nums and two integers, m and k . We must examine every contiguous subarray whose length is exactly k . Among those subarrays, we only consider the ones that contain at least m distinct values. These are called almost...

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LeetCode 2767 - Partition String Into Minimum Beautiful Substrings

LeetCode Problem 2767 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Backtracking Solution Problem Understanding The problem asks us to partition a given binary string s into the minimum number of substrings such that each substring is beautiful . A substring is beautiful if it represents a power of 5 in decimal and does not have leading zeros. The string s contains only '0' and '1' characters, and its...

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LeetCode 2524 - Maximum Frequency Score of a Subarray

LeetCode Problem 2524 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Stack, Sliding Window Solution Problem Understanding The problem asks us to compute the maximum frequency score among all contiguous subarrays of length k from a given integer array nums . A frequency score is defined as the sum of each distinct element raised to the power of its frequency within the subarray, taken modulo 10^9 + 7 . For...

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LeetCode 2605 - Form Smallest Number From Two Digit Arrays

LeetCode Problem 2605 Difficulty: 🟢 Easy Topics: Array, Hash Table, Enumeration Solution Problem Understanding The problem gives us two arrays, nums1 and nums2 , where each array contains unique digits from 1 to 9 . We need to construct the smallest possible number such that the number contains at least one digit from each array. The phrase "contains at least one digit from each array" is the key requirement. A...

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LeetCode 2713 - Maximum Strictly Increasing Cells in a Matrix

LeetCode Problem 2713 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Dynamic Programming, Memoization, Sorting, Matrix, Ordered Set Solution Problem Understanding The problem gives us an m x n matrix where each cell contains an integer value. We may start from any cell, and from the current cell we are allowed to move only within the same row or the same column. However, there is an important restriction: we...

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LeetCode 3146 - Permutation Difference between Two Strings

LeetCode Problem 3146 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to compute the permutation difference between two strings, s and t . Both strings contain unique characters, and t is a permutation of s . The permutation difference is calculated by taking each character in s , finding its index in both s and t , and summing the absolute differences of these...

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LeetCode 2262 - Total Appeal of A String

LeetCode Problem 2262 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming Solution Problem Understanding The problem asks us to calculate the total appeal of all substrings of a given string s . The appeal of a string is defined as the number of distinct characters it contains. In other words, for every possible substring of s , we need to count how many unique letters it has and sum...

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LeetCode 2301 - Match Substring After Replacement

LeetCode Problem 2301 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, String Matching Solution Problem Understanding The problem gives us a string s , a target string sub , and a list of character replacement rules called mappings . Each mapping [old, new] means that a character old inside sub may be replaced with new . The replacement is optional, and every character in sub can be replaced at most...

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LeetCode 2588 - Count the Number of Beautiful Subarrays

LeetCode Problem 2588 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation, Prefix Sum Solution Problem Understanding The problem gives us an array nums , and we are allowed to repeatedly perform a special operation on pairs of elements. In one operation, we choose two different indices i and j , and also choose a bit position k such that both nums[i] and nums[j] currently have the k th bit...

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LeetCode 3298 - Count Substrings That Can Be Rearranged to Contain a String II

LeetCode Problem 3298 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding We are given two strings, word1 and word2 . We need to count how many substrings of word1 are considered valid. A substring is valid if its characters can be rearranged so that word2 becomes a prefix of the rearranged string. This condition is equivalent to saying that the substring must contain at least all...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2657 - Find the Prefix Common Array of Two Arrays

LeetCode Problem 2657 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation Solution Problem Understanding The problem provides two arrays, A and B , each a permutation of integers from 1 to n . A permutation means each number from 1 to n appears exactly once in the array. We are asked to compute a new array C , called the prefix common array , where C[i] represents the number...

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LeetCode 2014 - Longest Subsequence Repeated k Times

LeetCode Problem 2014 Difficulty: 🔴 Hard Topics: Hash Table, Two Pointers, String, Backtracking, Counting, Enumeration Solution Problem Understanding The problem asks us to find the longest subsequence of a string s that can be repeated k times while still being a subsequence of s . A subsequence is derived by deleting zero or more characters from a string without changing the order of the remaining characters. A subsequence seq repeated...

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LeetCode 2870 - Minimum Number of Operations to Make Array Empty

LeetCode Problem 2870 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution LeetCode 2870 - Minimum Number of Operations to Make Array Empty Problem Understanding The problem gives us an array nums containing positive integers. In a single operation, we are allowed to remove either: Exactly 2 equal elements, or Exactly 3 equal elements. Our goal is to remove all elements from the array using the minimum possible number...

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LeetCode 2085 - Count Common Words With One Occurrence

LeetCode Problem 2085 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution LeetCode 2085, Count Common Words With One Occurrence Problem Understanding This problem gives us two arrays of strings, words1 and words2 . Our task is to count how many strings appear exactly once in both arrays. The key detail is that a word only qualifies if: It appears exactly one time in words1 It appears exactly one...

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LeetCode 3279 - Maximum Total Area Occupied by Pistons

LeetCode Problem 3279 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Simulation, Counting, Prefix Sum Solution Problem Understanding This problem is about simulating the motion of pistons in a car engine and computing the maximum total area under the pistons over time. Each piston moves either up or down by 1 unit every second, reversing direction when it hits the top ( height ) or bottom ( 0 ). The...

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LeetCode 2133 - Check if Every Row and Column Contains All Numbers

LeetCode Problem 2133 Difficulty: 🟢 Easy Topics: Array, Hash Table, Matrix Solution Problem Understanding In this problem, we are given an n x n matrix where every value is guaranteed to be between 1 and n . A matrix is considered valid if every row contains all integers from 1 through n exactly once, and every column also contains all integers from 1 through n exactly once. Another way to...

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LeetCode 2099 - Find Subsequence of Length K With the Largest Sum

LeetCode Problem 2099 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem asks us to find a subsequence of length k from a given integer array nums such that the sum of the elements in the subsequence is maximized. A subsequence is defined as a sequence that can be obtained from the original array by deleting zero or more elements without changing the...

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LeetCode 2424 - Longest Uploaded Prefix

LeetCode Problem 2424 Difficulty: 🟡 Medium Topics: Hash Table, Binary Search, Union-Find, Design, Binary Indexed Tree, Segment Tree, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to design a data structure that tracks uploaded videos and continuously reports the length of the longest uploaded prefix. A prefix of uploaded videos means that every video from 1 through i has already been uploaded. The goal is to...

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LeetCode 3078 - Match Alphanumerical Pattern in Matrix I

LeetCode Problem 3078 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Matrix Solution LeetCode 3078 - Match Alphanumerical Pattern in Matrix I Problem Understanding This problem asks us to search for a rectangular submatrix inside a larger integer matrix, where the submatrix follows a pattern described by digits and lowercase letters. The input consists of: board , a 2D matrix of integers from 0 to 9 pattern , a 2D...

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LeetCode 2354 - Number of Excellent Pairs

LeetCode Problem 2354 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Bit Manipulation Solution Problem Understanding The problem provides a 0-indexed array of positive integers nums and a positive integer k . The task is to count all distinct pairs (num1, num2) such that both numbers exist in nums and the sum of the number of set bits in num1 OR num2 and num1 AND num2 is at least...

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LeetCode 2225 - Find Players With Zero or One Losses

LeetCode Problem 2225 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem gives us a list of match results. Each match is represented as a pair: [winner, loser] This means the player winner defeated the player loser . Our goal is to return two separate lists: All players who never lost any match. All players who lost exactly one match. Both lists must be sorted...

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LeetCode 2008 - Maximum Earnings From Taxi

LeetCode Problem 2008 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Dynamic Programming, Sorting Solution Problem Understanding The problem describes a taxi traveling along a one directional road from point 1 to point n . Along the way, passengers request rides. Each ride is represented as [start, end, tip] , meaning the passenger wants to travel from start to end and will additionally pay a tip. If we accept...

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LeetCode 1994 - The Number of Good Subsets

LeetCode Problem 1994 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Dynamic Programming, Bit Manipulation, Counting, Number Theory, Bitmask Solution LeetCode 1994 - The Number of Good Subsets Problem Understanding This problem asks us to count how many subsets of the input array have a product that can be written as a multiplication of distinct prime numbers. The phrase "distinct prime numbers" is the key restriction. A product is valid...

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LeetCode 2456 - Most Popular Video Creator

LeetCode Problem 2456 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem gives us three parallel arrays: creators[i] represents the creator of the i th video ids[i] represents the video ID of the i th video views[i] represents the number of views for the i th video Each index corresponds to one video. Multiple videos can belong to the same creator, and...

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LeetCode 3327 - Check if DFS Strings Are Palindromes

LeetCode Problem 3327 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Tree, Depth-First Search, Hash Function Solution LeetCode 3327 - Check if DFS Strings Are Palindromes Problem Understanding We are given a rooted tree with n nodes. Node 0 is the root, and the parent-child relationships are described by the parent array. Each node i also has an associated character s[i] . For any node x , a DFS traversal...

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LeetCode 2840 - Check if Strings Can be Made Equal With Operations II

LeetCode Problem 2840 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting Solution Problem Understanding The problem asks us to determine whether two strings s1 and s2 of equal length can be made identical using a specific type of swap operation. The operation allows us to choose any two indices i and j such that i < j and j - i is even , and then swap the characters at...

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LeetCode 2342 - Max Sum of a Pair With Equal Sum of Digits

LeetCode Problem 2342 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem requires finding the maximum sum of a pair of numbers in an array such that the sum of the digits of both numbers is equal. Specifically, you are given a 0-indexed array nums containing positive integers. You can choose two distinct indices i and j such that i != j ,...

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LeetCode 3081 - Replace Question Marks in String to Minimize Its Value

LeetCode Problem 3081 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The problem gives us a string s containing lowercase English letters and the character '?' . Every '?' must be replaced with a lowercase letter so that the resulting string has the minimum possible value. The value of a string is determined by counting repeated occurrences of letters. For every position...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2799 - Count Complete Subarrays in an Array

LeetCode Problem 2799 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution Problem Understanding The problem asks us to count the number of complete subarrays in a given array nums . A subarray is complete if it contains all distinct elements that exist in the entire array. In other words, if the total number of distinct elements in nums is k , then a complete subarray must also have...

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LeetCode 2641 - Cousins in Binary Tree II

LeetCode Problem 2641 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding The problem gives us the root of a binary tree and asks us to replace every node’s value with the sum of all of its cousins’ values. Two nodes are considered cousins if: They are at the same depth in the tree They do not share the same parent The depth...

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LeetCode 2831 - Find the Longest Equal Subarray

LeetCode Problem 2831 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Sliding Window Solution Problem Understanding The problem asks us to find the longest contiguous subarray where all elements are equal , after we are allowed to delete at most k elements from the original array. In simpler terms, we are trying to create the longest block of identical numbers by optionally removing some disruptive numbers, up to k...

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LeetCode 2306 - Naming a Company

LeetCode Problem 2306 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Bit Manipulation, Enumeration Solution Problem Understanding The problem gives us a list of unique strings called ideas . We must repeatedly choose two different strings, swap their first characters, and determine whether the newly formed strings are both absent from the original list. If both newly generated names do not already exist in ideas , then the ordered pair...

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LeetCode 2856 - Minimum Array Length After Pair Removals

LeetCode Problem 2856 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Binary Search, Greedy, Counting Solution Problem Understanding This problem gives us a sorted integer array nums in non-decreasing order. We may repeatedly remove pairs of elements under one important rule: for a chosen pair (i, j) , the values must satisfy nums[i] < nums[j] . In other words, the two chosen numbers cannot be equal, and the smaller...

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LeetCode 2336 - Smallest Number in Infinite Set

LeetCode Problem 2336 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to implement a data structure representing an infinite set of positive integers, initially containing all integers starting from 1. We need to provide two main operations: popSmallest , which removes and returns the smallest integer in the set, and addBack , which adds a previously removed integer back...

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LeetCode 1940 - Longest Common Subsequence Between Sorted Arrays

LeetCode Problem 1940 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find the longest common subsequence (LCS) among multiple sorted integer arrays. In simpler terms, we are given a list of arrays where each array is sorted in strictly increasing order, and we need to identify the sequence of numbers that appears in all arrays in the same relative order and is...

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LeetCode 2423 - Remove Letter To Equalize Frequency

LeetCode Problem 2423 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine if we can remove exactly one character from a string word such that all remaining characters in the string have the same frequency . The input string consists of lowercase English letters and has a length between 2 and 100. The output is a boolean indicating whether this transformation is...

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LeetCode 2489 - Number of Substrings With Fixed Ratio

LeetCode Problem 2489 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Prefix Sum Solution Problem Understanding The problem gives us a binary string s , along with two coprime integers num1 and num2 . We need to count how many non-empty substrings contain 0 s and 1 s in the exact ratio num1 : num2 . If a substring contains: zeroCount zeros oneCount ones then it is valid only if:...

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LeetCode 3134 - Find the Median of the Uniqueness Array

LeetCode Problem 3134 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Sliding Window Solution Problem Understanding The problem defines a special array called the uniqueness array. For every possible subarray of nums , we compute how many distinct values appear inside that subarray. We then collect all of those distinct counts into a single array and sort it in non decreasing order. The task is to return the median...

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LeetCode 3005 - Count Elements With Maximum Frequency

LeetCode Problem 3005 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution LeetCode 3005 - Count Elements With Maximum Frequency Problem Understanding The problem gives us an integer array nums , where every value is positive. We need to determine which elements appear most frequently, then return the total number of occurrences contributed by all such elements. In other words, we first compute the frequency of every distinct number in...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2364 - Count Number of Bad Pairs

LeetCode Problem 2364 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to count the number of bad pairs in an array. We are given a 0-indexed integer array nums , and a pair of indices (i, j) is considered bad if: i < j j - i != nums[j] - nums[i] In other words, for every pair of indices where the first...

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LeetCode 2227 - Encrypt and Decrypt Strings

LeetCode Problem 2227 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to implement an Encrypter class that can both encrypt and decrypt strings according to custom character mappings. The keys array provides the characters that can be encrypted, and values provides the corresponding 2-character strings that each key maps to. The encryption of a string is straightforward: each character is replaced...

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LeetCode 2581 - Count Number of Possible Root Nodes

LeetCode Problem 2581 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Tree, Depth-First Search Solution Problem Understanding This problem gives us an undirected tree with n nodes and a list of directed parent-child guesses. Each guess claims that one node is the parent of another node when the tree is rooted in some way. The important detail is that the tree itself is undirected. Parent-child relationships only become defined...

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LeetCode 2441 - Largest Positive Integer That Exists With Its Negative

LeetCode Problem 2441 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Sorting Solution Problem Understanding The problem asks us to find the largest positive integer k in a given array nums such that its negative counterpart -k also exists in the array. In other words, for each positive integer in the array, we need to check whether its negative exists. If multiple positive integers satisfy this property, we return...

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LeetCode 2461 - Maximum Sum of Distinct Subarrays With Length K

LeetCode Problem 2461 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution That prompt contains two different LeetCode problems merged together, but the second problem, LeetCode 2461 - Maximum Sum of Distinct Subarrays With Length K , appears to be the target because the detailed sections and code stubs at the end correspond to it. I will provide the full solution guide for LeetCode 2461 . LeetCode 2461 -...

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LeetCode 2251 - Number of Flowers in Full Bloom

LeetCode Problem 2251 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Sorting, Prefix Sum, Ordered Set Solution Problem Understanding The problem asks us to determine, for a list of people arriving at certain times, how many flowers are in full bloom at the time of their arrival. Each flower has a start and end time representing the period it is in full bloom, inclusive of both endpoints. The input...

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LeetCode 2201 - Count Artifacts That Can Be Extracted

LeetCode Problem 2201 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution Problem Understanding This problem models an excavation process on a square n x n grid. Several rectangular artifacts are buried in the grid, and each artifact occupies one or more cells. Every artifact is described by four integers: (r1, c1) represents the top-left corner (r2, c2) represents the bottom-right corner This means the artifact occupies every cell inside...

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LeetCode 2131 - Longest Palindrome by Concatenating Two Letter Words

LeetCode Problem 2131 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Greedy, Counting Solution Problem Understanding This problem asks us to construct the longest possible palindrome using a list of two-letter words. Each word can be used at most once, and the order of concatenation can be chosen freely. A palindrome reads the same forwards and backwards, which means that for a word pair to appear symmetrically around the center,...

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LeetCode 2593 - Find Score of an Array After Marking All Elements

LeetCode Problem 2593 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Heap (Priority Queue), Simulation Solution Here is a comprehensive, detailed technical solution guide for LeetCode 2593 - Find Score of an Array After Marking All Elements , following your requested format precisely. Problem Understanding The problem provides an array of positive integers, nums . We are asked to simulate a scoring process where at each step we select the...

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LeetCode 2537 - Count the Number of Good Subarrays

LeetCode Problem 2537 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution

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LeetCode 3092 - Most Frequent IDs

LeetCode Problem 3092 Difficulty: 🟡 Medium Topics: Array, Hash Table, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to maintain a collection of IDs that is updated incrementally based on two arrays, nums and freq . Each index i represents a step. At step i , we either add freq[i] copies of nums[i] to the collection if freq[i] is positive, or remove -freq[i] copies of nums[i]...

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LeetCode 2357 - Make Array Zero by Subtracting Equal Amounts

LeetCode Problem 2357 Difficulty: 🟢 Easy Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Simulation Solution LeetCode 2357 - Make Array Zero by Subtracting Equal Amounts Problem Understanding This problem gives us an array nums containing non-negative integers. We want to transform the entire array into zeros using a specific operation. In a single operation, we must choose a positive integer x that is less than or equal to...

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LeetCode 2584 - Split the Array to Make Coprime Products

LeetCode Problem 2584 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Number Theory Solution Problem Understanding The problem is asking us to find a valid split point in an array nums where the product of elements to the left of the split and the product of elements to the right are coprime . Formally, if we split at index i , the left product is nums[0] * nums[1] * ......

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LeetCode 2958 - Length of Longest Subarray With at Most K Frequency

LeetCode Problem 2958 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest contiguous subarray in an array of integers nums such that the frequency of every element in that subarray does not exceed a given integer k . In other words, in the resulting subarray, no number appears more than k times. The input consists of...

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LeetCode 3120 - Count the Number of Special Characters I

LeetCode Problem 3120 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to count the number of special characters in a given string word . A character is defined as special if it appears in both lowercase and uppercase forms within the same string. For example, if 'a' appears as both 'a' and 'A' anywhere in word , then 'a' counts as a special character....

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LeetCode 2913 - Subarrays Distinct Element Sum of Squares I

LeetCode Problem 2913 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to examine every possible non-empty subarray of the given array nums . For each subarray, we compute how many distinct values appear inside it. After finding this distinct count, we square it and add it to the final answer. A subarray is contiguous, which means the elements must appear next to each other...

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LeetCode 2249 - Count Lattice Points Inside a Circle

LeetCode Problem 2249 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Geometry, Enumeration Solution Problem Understanding The problem asks us to determine how many lattice points lie inside at least one of a set of circles on a 2D grid. Each circle is represented by a triplet [xi, yi, ri] , where (xi, yi) is the circle's center and ri is its radius. A lattice point is any point on...

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LeetCode 2875 - Minimum Size Subarray in Infinite Array

LeetCode Problem 2875 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window, Prefix Sum Solution LeetCode 2875 - Minimum Size Subarray in Infinite Array Problem Understanding This problem gives us a finite array nums , but asks us to imagine an infinite array called infinite_nums created by repeating nums forever. For example: nums = [1, 2, 3] infinite_nums = [1,2,3,1,2,3,1,2,3,1,2,3,...] We must find the shortest contiguous subarray in this infinite...

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LeetCode 2196 - Create Binary Tree From Descriptions

LeetCode Problem 2196 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Binary Tree Solution Problem Understanding The problem asks us to construct a binary tree from a list of relationships, where each relationship is given as [parent, child, isLeft] . Here, parent is the value of a node, child is the value of its direct descendant, and isLeft indicates whether child is a left child ( 1 ) or a...

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LeetCode 3267 - Count Almost Equal Pairs II

LeetCode Problem 3267 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sorting, Counting, Enumeration Solution LeetCode 3267 - Count Almost Equal Pairs II Problem Understanding We are given an array nums containing positive integers. We need to count how many pairs of indices (i, j) with i < j satisfy a special condition called almost equal . Two numbers x and y are considered almost equal if we can make them...

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LeetCode 2385 - Amount of Time for Binary Tree to Be Infected

LeetCode Problem 2385 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding This problem asks us to model the spread of an infection across a binary tree. Each node in the tree has a unique integer value, and the infection starts from a specific node, identified by start . The infection spreads to adjacent nodes-this means both the parent and child nodes of...

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LeetCode 2526 - Find Consecutive Integers from a Data Stream

LeetCode Problem 2526 Difficulty: 🟡 Medium Topics: Hash Table, Design, Queue, Counting, Data Stream Solution Problem Understanding The problem asks us to design a data structure that continuously processes a stream of integers and determines whether the most recent k integers are all equal to a specific target value. We are given two parameters when initializing the data structure: value , which represents the integer we are interested in tracking....

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LeetCode 2747 - Count Zero Request Servers

LeetCode Problem 2747 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window, Sorting Solution Problem Understanding This problem asks us to count servers that did not receive any requests within a certain time window for multiple queries. You are given n servers, each with a unique ID from 1 to n . The logs array records requests, where each entry [server_id, time] indicates that server_id received a request at time...

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LeetCode 2121 - Intervals Between Identical Elements

LeetCode Problem 2121 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution LeetCode 2121 - Intervals Between Identical Elements Problem Statement You are given a 0-indexed integer array arr . For every index i , you must compute the sum of distances between i and every other index j where arr[i] == arr[j] . The distance between two indices is the absolute difference of their positions: $$|i - j|$$...

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LeetCode 2266 - Count Number of Texts

LeetCode Problem 2266 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Dynamic Programming Solution Problem Understanding The problem requires determining how many possible text messages Alice could have sent given a sequence of digit key presses received by Bob. Each digit from '2' to '9' maps to a set of letters on a phone keypad. Pressing a key multiple times selects a different letter on that key. For example, pressing...

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LeetCode 2512 - Reward Top K Students

LeetCode Problem 2512 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem asks us to compute a ranking of students based on textual feedback they receive. We are given two lists of words: positive_feedback and negative_feedback . Every time a student receives a report, we score that report by adding 3 points for each word that appears in positive_feedback and subtracting 1...

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LeetCode 2499 - Minimum Total Cost to Make Arrays Unequal

LeetCode Problem 2499 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding You are given two arrays, nums1 and nums2 , both of length n . You are allowed to perform operations only on nums1 . In a single operation, you may swap any two indices in nums1 , and the cost of that operation is the sum of the two indices involved in the swap. The...

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LeetCode 2682 - Find the Losers of the Circular Game

LeetCode Problem 2682 Difficulty: 🟢 Easy Topics: Array, Hash Table, Simulation Solution Problem Understanding This problem describes a circular passing game among n friends numbered from 1 to n . The ball always starts with friend 1 , and each turn increases the number of clockwise steps by a multiple of k . More specifically: On the first turn, the current player passes the ball 1 * k steps clockwise....

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LeetCode 2707 - Extra Characters in a String

LeetCode Problem 2707 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Dynamic Programming, Trie Solution Problem Understanding The problem asks us to break a string s into non-overlapping substrings such that each substring exists in a given dictionary . Characters in s that cannot be matched with any dictionary word are considered extra characters . The goal is to find the minimum number of extra characters when splitting s optimally....

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LeetCode 2295 - Replace Elements in an Array

LeetCode Problem 2295 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution Problem Understanding The problem asks us to perform a series of replacement operations on an array of distinct integers. Specifically, we have an initial array nums containing n distinct positive integers. We are also given m operations, each consisting of a pair [oldValue, newValue] . For each operation, we must locate the element oldValue in the array and...

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LeetCode 2182 - Construct String With Repeat Limit

LeetCode Problem 2182 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Heap (Priority Queue), Counting Solution Problem Understanding The problem asks us to construct a string from the characters of a given string s under a specific constraint: no character can appear more than repeatLimit times consecutively. The goal is not just to create any valid string but to produce the lexicographically largest one possible. Lexicographic ordering is similar to...

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LeetCode 2808 - Minimum Seconds to Equalize a Circular Array

LeetCode Problem 2808 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem gives a circular array nums of length n . At each second, every element in the array can be replaced simultaneously by either itself, its previous neighbor, or its next neighbor. The task is to determine the minimum number of seconds required to make all elements equal. In simpler terms, the array elements "propagate" their...

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LeetCode 1993 - Operations on Tree

LeetCode Problem 1993 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Breadth-First Search, Design Solution LeetCode 1993 - Operations on Tree Problem Understanding The problem asks us to design a mutable tree-based data structure that supports three operations: lock , unlock , and upgrade . Each node in the tree may either be unlocked or locked by exactly one user. The operations must follow strict rules about when...

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LeetCode 2508 - Add Edges to Make Degrees of All Nodes Even

LeetCode Problem 2508 Difficulty: 🔴 Hard Topics: Hash Table, Graph Theory Solution Problem Understanding The problem gives us an undirected graph with n nodes and a list of edges. Each edge connects two different nodes, and the graph may contain multiple disconnected components. Our task is to determine whether it is possible to add at most two new edges so that every node in the graph ends up having an...

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LeetCode 3272 - Find the Count of Good Integers

LeetCode Problem 3272 Difficulty: 🔴 Hard Topics: Hash Table, Math, Combinatorics, Enumeration Solution Problem Understanding The problem asks us to count n-digit integers that are "good" with respect to a given integer k . A "good" integer is one whose digits can be rearranged to form a k-palindromic integer . A k-palindromic integer is defined as an integer that satisfies two properties: it is a palindrome (reads the same forwards...

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LeetCode 3316 - Find Maximum Removals From Source String

LeetCode Problem 3316 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Dynamic Programming Solution LeetCode 3316 - Find Maximum Removals From Source String Problem Understanding We are given three inputs: source , the original string of length n pattern , a string that is guaranteed to already be a subsequence of source targetIndices , a sorted list of indices in source that are eligible for removal An operation...

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LeetCode 2368 - Reachable Nodes With Restrictions

LeetCode Problem 2368 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Breadth-First Search, Union-Find, Graph Theory Solution That is a large, highly structured guide. To make sure I target the exact problem and keep the response complete in a single message, can you confirm you want the full solution guide for LeetCode 2368 - Reachable Nodes With Restrictions ? The prompt body currently says 2368 , while some...

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LeetCode 3369 - Design an Array Statistics Tracker

LeetCode Problem 3369 Difficulty: 🔴 Hard Topics: Hash Table, Binary Search, Design, Queue, Heap (Priority Queue), Data Stream, Ordered Set Solution Problem Understanding The problem asks us to design a data structure that supports a stream of operations on a dynamic collection of integers. Numbers are added over time, and the oldest inserted number can also be removed. At any point, we must efficiently answer three statistical queries: The floored...

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LeetCode 2964 - Number of Divisible Triplet Sums

LeetCode Problem 2964 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution LeetCode 2964 - Number of Divisible Triplet Sums Problem Understanding The problem gives us an integer array nums and an integer d . We must count how many triplets of indices (i, j, k) satisfy two conditions: The indices are strictly increasing, meaning i < j < k . The sum of the corresponding elements is divisible by d...

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LeetCode 3391 - Design a 3D Binary Matrix with Efficient Layer Tracking

LeetCode Problem 3391 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Heap (Priority Queue), Matrix, Ordered Set Solution Problem Understanding The problem asks us to design a data structure that manages a 3D binary matrix of size n x n x n . Every cell initially contains 0 , and we must support three operations efficiently: setCell(x, y, z) sets a specific cell to 1 . unsetCell(x, y, z) sets...

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LeetCode 3318 - Find X-Sum of All K-Long Subarrays I

LeetCode Problem 3318 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sliding Window, Heap (Priority Queue) Solution Problem Understanding The problem asks us to compute a special value called the x-sum for every contiguous subarray of length k . For each window of size k , we first count how many times each number appears. After that, we only keep the contributions of the top x most frequent distinct values. If...

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LeetCode 2947 - Count Beautiful Substrings I

LeetCode Problem 2947 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Enumeration, Number Theory, Prefix Sum Solution LeetCode 2947 - Count Beautiful Substrings I Problem Understanding We are given a lowercase English string s and a positive integer k . We must count how many non-empty substrings satisfy the definition of a beautiful substring. For a substring, let: vowels be the number of vowel characters ( a , e ,...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2612 - Minimum Reverse Operations

LeetCode Problem 2612 Difficulty: 🔴 Hard Topics: Array, Hash Table, Breadth-First Search, Union-Find, Ordered Set Solution This will be a very large, multi-thousand-word technical guide because of the required depth, full proofs, worked examples, Python and Go implementations, detailed walkthroughs, complexity reasoning, and exhaustive tests. To ensure quality and avoid truncation, I will split it cleanly into sections across messages. Part 1: Problem Understanding, Approaches, Algorithm Walkthrough, and Python Solution...

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LeetCode 3085 - Minimum Deletions to Make String K-Special

LeetCode Problem 3085 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Counting Solution Problem Understanding The problem asks us to transform a given string word into a k-special string using the minimum number of deletions. A string is considered k-special if, for every pair of characters in the string, the difference in their frequencies does not exceed k . In simpler terms, all characters' counts must be roughly balanced,...

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LeetCode 3337 - Total Characters in String After Transformations II

LeetCode Problem 3337 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Dynamic Programming, Counting Solution Problem Understanding The problem defines a repeated string transformation process over lowercase English letters. Every character does not simply become one new character, instead it expands into multiple characters depending on the value stored in nums . For a character c , let its alphabet index be: 'a' -> 0 'b' -> 1 ... 'z'...

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LeetCode 2768 - Number of Black Blocks

LeetCode Problem 2768 Difficulty: 🟡 Medium Topics: Array, Hash Table, Enumeration Solution Problem Understanding This problem asks us to count the number of 2 x 2 blocks in a grid based on how many black cells they contain. We are given the dimensions of a grid, m rows and n columns, and a list of coordinates representing black cells. Every cell not listed is white. A block is a 2...

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LeetCode 1876 - Substrings of Size Three with Distinct Characters

LeetCode Problem 1876 Difficulty: 🟢 Easy Topics: Hash Table, String, Sliding Window, Counting Solution Problem Understanding The problem asks us to count how many substrings of length exactly three contain only distinct characters. A substring is a continuous portion of the string, so for every position in the string, we can examine the next three consecutive characters and determine whether all three are different. For example, in the string "xyzzaz"...

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LeetCode 2350 - Shortest Impossible Sequence of Rolls

LeetCode Problem 2350 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy Solution Here is a complete, detailed technical solution guide for LeetCode 2350 - Shortest Impossible Sequence of Rolls , following your requested format precisely. Problem Understanding The problem asks us to determine the length of the shortest sequence of dice rolls that cannot be found as a subsequence within a given list of dice rolls. We are given an...

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LeetCode 2744 - Find Maximum Number of String Pairs

LeetCode Problem 2744 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Simulation Solution LeetCode 2744 - Find Maximum Number of String Pairs Problem Understanding This problem gives us an array words containing distinct strings. Every string has length exactly 2 , and all strings consist of lowercase English letters. We can form a pair between two strings words[i] and words[j] when: i < j One string is exactly the reverse...

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LeetCode 3289 - The Two Sneaky Numbers of Digitville

LeetCode Problem 3289 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math Solution Problem Understanding The problem is asking us to identify two numbers that appear twice in an otherwise consecutive list of integers ranging from 0 to n - 1 . In other words, the input nums is almost a permutation of the numbers 0 through n - 1 , except that two numbers are repeated , so the length...

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LeetCode 3217 - Delete Nodes From Linked List Present in Array

LeetCode Problem 3217 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List Solution Problem Understanding This problem asks us to modify a singly linked list by removing all nodes whose values appear in a given array nums . The input consists of two elements: an array of integers nums and the head of a linked list. Each node in the linked list contains an integer value and a pointer to...

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LeetCode 3329 - Count Substrings With K-Frequency Characters II

LeetCode Problem 3329 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution LeetCode 3329 - Count Substrings With K-Frequency Characters II Problem Understanding The problem asks us to count how many substrings of a given string s contain at least one character that appears at least k times within that substring. A substring is a contiguous portion of the string. For every possible substring, we need to determine whether...

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LeetCode 3242 - Design Neighbor Sum Service

LeetCode Problem 3242 Difficulty: 🟢 Easy Topics: Array, Hash Table, Design, Matrix, Simulation Solution Problem Understanding The problem asks us to design a service that works on a square matrix, where every value in the matrix is unique. For any given value, we must efficiently compute two different types of neighbor sums. The first operation, adjacentSum(value) , returns the sum of all directly adjacent cells. These are the four cardinal...

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LeetCode 2670 - Find the Distinct Difference Array

LeetCode Problem 2670 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to compute the distinct difference array for a given integer array nums . For each index i in the array, we calculate the difference between the number of distinct elements in the prefix nums[0, ..., i] and the number of distinct elements in the suffix nums[i + 1, ..., n - 1] ....

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LeetCode 3160 - Find the Number of Distinct Colors Among the Balls

LeetCode Problem 3160 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution Problem Understanding The problem gives us limit + 1 balls labeled from 0 to limit . Initially, none of the balls have a color assigned. We then process a sequence of queries, where each query is of the form [x, y] . This means ball x should now be painted with color y . The important detail is...

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LeetCode 3035 - Maximum Palindromes After Operations

LeetCode Problem 3035 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Greedy, Sorting, Counting Solution LeetCode 3035 - Maximum Palindromes After Operations Problem Understanding This problem gives us an array of strings, words , and allows us to swap any character with any other character across the entire collection of strings. The swaps are completely unrestricted. We may swap characters between different strings, within the same string, or perform no...

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LeetCode 2442 - Count Number of Distinct Integers After Reverse Operations

LeetCode Problem 2442 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to compute the number of distinct integers in an array after performing a specific operation on each element: reversing its digits and adding it to the array. In other words, for every integer in nums , we generate its reverse and append it. After processing all original elements, we count how...

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LeetCode 2152 - Minimum Number of Lines to Cover Points

LeetCode Problem 2152 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Dynamic Programming, Backtracking, Bit Manipulation, Geometry, Bitmask Solution Problem Understanding This problem asks us to determine the minimum number of straight lines needed to cover a given set of points on an X-Y plane. Each point is defined by its (x, y) coordinates, and a straight line can pass through any number of points as long as they are...

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LeetCode 2103 - Rings and Rods

LeetCode Problem 2103 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem is asking us to determine how many rods, labeled from 0 to 9, have all three colors of rings placed on them. Each ring is described by a two-character string: the first character is the color ( 'R' for red, 'G' for green, 'B' for blue) and the second character is the rod index (...

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LeetCode 2606 - Find the Substring With Maximum Cost

LeetCode Problem 2606 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Dynamic Programming Solution Problem Understanding The problem asks us to find the substring of a given string s that maximizes a custom cost function. Each character in the string has a value: if it exists in the string chars , its value is taken from the corresponding index in the array vals ; if it does not exist in...

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LeetCode 2019 - The Score of Students Solving Math Expression

LeetCode Problem 2019 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, String, Dynamic Programming, Stack, Memoization Solution LeetCode 2019, The Score of Students Solving Math Expression Problem Understanding The problem gives us a mathematical expression string s containing only: Single digit numbers 0-9 Addition operators + Multiplication operators * The expression is guaranteed to be valid. For example: 3+5*2 7+3*1*2 Students were supposed to evaluate the expression using the correct...

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LeetCode 3167 - Better Compression of String

LeetCode Problem 3167 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting, Counting Solution Problem Understanding The problem gives us a compressed string where every character is immediately followed by its frequency. For example, the string "a3b2" represents the original expanded string "aaabb" . However, the input compression is not guaranteed to be optimal. The same character may appear multiple times in different parts of the compressed string. For example: "a3c9b2c1"...

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LeetCode 2341 - Maximum Number of Pairs in Array

LeetCode Problem 2341 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to take an array of integers, nums , and repeatedly form pairs of equal numbers until no more pairs can be formed. Each operation removes exactly two identical numbers from the array, and we continue doing this until it is no longer possible. The goal is to return a list of two...

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LeetCode 3153 - Sum of Digit Differences of All Pairs

LeetCode Problem 3153 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to compute the total digit difference across every pair of numbers in the array. All numbers have the same number of digits. For any two numbers, their digit difference is defined as the number of positions where the digits are different. For example: 13 and 23 differ only at the first...

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LeetCode 1938 - Maximum Genetic Difference Query

LeetCode Problem 1938 Difficulty: 🔴 Hard Topics: Array, Hash Table, Bit Manipulation, Depth-First Search, Trie Solution Problem Understanding The problem presents a rooted tree where each node is uniquely identified by an integer from 0 to n-1 . This integer also represents the genetic value of the node. The tree structure is given by the parents array, where parents[i] indicates the parent of node i , and -1 indicates the...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2597 - The Number of Beautiful Subsets

LeetCode Problem 2597 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Dynamic Programming, Backtracking, Sorting, Combinatorics Solution Problem Understanding The problem asks us to count how many non-empty subsets of the array nums are considered "beautiful". A subset is beautiful if there are no two numbers inside the subset whose absolute difference equals k . For example, if k = 2 , then the subset cannot contain both 2 and...

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LeetCode 3389 - Minimum Operations to Make Character Frequencies Equal

LeetCode Problem 3389 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming, Counting, Enumeration Solution Problem Understanding The problem requires transforming a given string s into a good string , where a good string is defined as one in which every character appears the same number of times. The operations allowed include deleting a character, inserting a character, or incrementing a character to the next letter in the alphabet (with...

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LeetCode 2564 - Substring XOR Queries

LeetCode Problem 2564 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Bit Manipulation Solution Problem Understanding The problem gives us a binary string s and a list of queries. Each query contains two integers, first and second . For every query, we need to find a substring of s whose decimal value satisfies: $$val \oplus first = second$$ where ⊕ represents the bitwise XOR operation. We can rearrange the equation...

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LeetCode 3186 - Maximum Total Damage With Spell Casting

LeetCode Problem 3186 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Binary Search, Dynamic Programming, Sorting, Counting Solution Problem Understanding The problem gives us an array power , where each element represents the damage value of a spell. Every spell can be used at most once, and multiple spells may share the same damage value. The restriction is the important part of the problem. If we decide to cast...

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LeetCode 3002 - Maximum Size of a Set After Removals

LeetCode Problem 3002 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy Solution Problem Understanding The problem is asking us to maximize the number of unique elements we can have in a set after removing exactly half of the elements from two arrays nums1 and nums2 . Each array is of even length n , and after removing n/2 elements from each, we insert all remaining elements into a set. Since...

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LeetCode 2122 - Recover the Original Array

LeetCode Problem 2122 Difficulty: 🔴 Hard Topics: Array, Hash Table, Two Pointers, Sorting, Enumeration Solution Problem Understanding The problem presents a scenario in which Alice has an original array arr of length n consisting of positive integers. She chooses a positive integer k and generates two new arrays: lower and higher . The array lower is formed by subtracting k from each element of arr , while higher is formed...

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LeetCode 2349 - Design a Number Container System

LeetCode Problem 2349 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution LeetCode 2349 - Design a Number Container System Problem Understanding The problem asks us to design a data structure that supports two operations efficiently: Assign a number to a given index. Find the smallest index currently assigned to a given number. The important detail is that the change(index, number) operation can both insert and...

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LeetCode 2273 - Find Resultant Array After Removing Anagrams

LeetCode Problem 2273 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Sorting Solution Problem Understanding This problem asks us to repeatedly remove words from an array when two adjacent words are anagrams of each other . We are given a 0-indexed string array words , where every string contains only lowercase English letters. During each operation, we may choose an index i such that the word at words[i] is an...

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LeetCode 3076 - Shortest Uncommon Substring in an Array

LeetCode Problem 3076 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding The problem asks us to find, for each string in a given array, the shortest substring that does not appear in any other string in the array. If multiple shortest substrings exist, we must choose the lexicographically smallest one. If no such substring exists, the answer for that string is an empty string. In other...

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LeetCode 2062 - Count Vowel Substrings of a String

LeetCode Problem 2062 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem requires counting the number of vowel substrings in a given string word . A substring is any contiguous sequence of characters within the string. A vowel substring is defined as a substring that contains only vowels ( 'a' , 'e' , 'i' , 'o' , 'u' ) and must include all five vowels at least...

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LeetCode 2351 - First Letter to Appear Twice

LeetCode Problem 2351 Difficulty: 🟢 Easy Topics: Hash Table, String, Bit Manipulation, Counting Solution Problem Understanding The problem gives us a string s containing only lowercase English letters. We need to return the first letter whose second occurrence appears earliest in the string. This detail is extremely important. The problem is not asking for the first character that repeats somewhere later. Instead, it asks which character reaches its second appearance...

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LeetCode 2763 - Sum of Imbalance Numbers of All Subarrays

LeetCode Problem 2763 Difficulty: 🔴 Hard Topics: Array, Hash Table, Enumeration Solution LeetCode 2763 - Sum of Imbalance Numbers of All Subarrays Problem Understanding For any array, its imbalance number is determined after sorting the elements. Suppose we have a subarray and sort it into sarr . We examine every adjacent pair in the sorted order. Whenever the difference between consecutive values is greater than 1 , that pair contributes...

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LeetCode 1995 - Count Special Quadruplets

LeetCode Problem 1995 Difficulty: 🟢 Easy Topics: Array, Hash Table, Enumeration Solution Problem Understanding The problem asks us to count the number of distinct quadruplets (a, b, c, d) in a given array nums such that the sum of the first three elements equals the fourth, i.e., nums[a] + nums[b] + nums[c] == nums[d] , and the indices satisfy a < b < c < d . In simpler terms,...

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LeetCode 2157 - Groups of Strings

LeetCode Problem 2157 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Bit Manipulation, Union-Find Solution Problem Understanding The problem asks us to group a list of unique lowercase strings based on a special notion of connectivity. Each string can be represented as a set of letters , and two strings are connected if one can be transformed into the other with exactly one operation : adding a letter, removing a...

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LeetCode 3046 - Split the Array

LeetCode Problem 3046 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to determine if an even-length array can be split into two equal-sized subarrays, each containing only distinct elements . In other words, we must partition the array nums of length n into nums1 and nums2 of size n/2 such that there are no repeated numbers within either subarray. The function should return...

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LeetCode 1857 - Largest Color Value in a Directed Graph

LeetCode Problem 1857 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming, Graph Theory, Topological Sort, Memoization, Counting Solution Problem Understanding The problem asks us to find the largest color value along any path in a directed graph of n nodes, where each node has a color represented by a lowercase English letter. The graph may contain cycles, in which case the function should return -1 . The input consists...

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LeetCode 2206 - Divide Array Into Equal Pairs

LeetCode Problem 2206 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation, Counting Solution Problem Understanding The problem gives us an integer array nums containing exactly 2 * n elements. Our task is to determine whether it is possible to split the array into n valid pairs, where each pair contains two identical numbers. In simpler terms, every number in the array must be matched with another occurrence of the...

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LeetCode 3371 - Identify the Largest Outlier in an Array

LeetCode Problem 3371 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting, Enumeration Solution Problem Understanding The problem gives us an integer array nums containing exactly three types of elements: n - 2 special numbers One element equal to the sum of all special numbers One outlier The task is to return the largest possible value that could serve as the outlier. The important detail is that these roles are determined...

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LeetCode 2842 - Count K-Subsequences of a String With Maximum Beauty

LeetCode Problem 2842 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Greedy, Sorting, Combinatorics Solution Problem Understanding The problem requires us to count k-subsequences of a string s that have the maximum beauty , where beauty is defined as the sum of the frequency of each character in the subsequence. A k-subsequence is a subsequence of length k where all characters are unique. The input string can be up to...

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LeetCode 2370 - Longest Ideal Subsequence

LeetCode Problem 2370 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming Solution Problem Understanding In this problem, we are given a string s containing only lowercase English letters and an integer k . We need to find the length of the longest subsequence such that the difference between every pair of adjacent characters in that subsequence is at most k . A subsequence does not require characters to be...

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LeetCode 2365 - Task Scheduler II

LeetCode Problem 2365 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution Problem Understanding The problem gives us an array tasks , where each value represents a task type. The tasks must be completed strictly in the given order, which means we cannot rearrange them to optimize the schedule. We are also given an integer space . After completing a task of a certain type, we must wait at least...

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LeetCode 3090 - Maximum Length Substring With Two Occurrences

LeetCode Problem 3090 Difficulty: 🟢 Easy Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem is asking for the maximum length of a substring in a given string s such that no character occurs more than twice within that substring. In other words, for any valid substring, each character can appear at most two times. You need to return the length of the longest such substring. The input...

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LeetCode 2043 - Simple Bank System

LeetCode Problem 2043 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Simulation Solution Problem Understanding This problem asks us to design a very simple banking system that supports three operations: Transfer money between two accounts Deposit money into an account Withdraw money from an account The bank contains n accounts numbered from 1 to n . The initial balance for each account is given in a 0-indexed array named balance...

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LeetCode 2260 - Minimum Consecutive Cards to Pick Up

LeetCode Problem 2260 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution Problem Understanding The problem gives an integer array cards , where each number represents the value written on a card. Two cards are considered matching if they contain the same value. We must find the minimum number of consecutive cards that need to be picked so that the selected group contains at least one matching pair. In...

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LeetCode 2384 - Largest Palindromic Number

LeetCode Problem 2384 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Counting Solution Problem Understanding The problem asks us to form the largest palindromic integer using the digits from a given string num . A palindrome is a number that reads the same forwards and backwards, like 121 or 7449447 . The input string num consists only of digits ( 0 to 9 ) and may include repeated digits. We...

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LeetCode 3159 - Find Occurrences of an Element in an Array

LeetCode Problem 3159 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem gives us three inputs: An integer array nums An integer array queries An integer x We need to answer each query independently. A query asks for the index of the kth occurrence of the value x inside the array nums . For example, if: nums = [1,3,1,7] x = 1 then the occurrences of 1...

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LeetCode 2115 - Find All Possible Recipes from Given Supplies

LeetCode Problem 2115 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Graph Theory, Topological Sort Solution Problem Understanding This problem asks us to determine which recipes can be created when we start with a set of available supplies and are allowed to recursively create additional recipes. Each recipe has a list of required ingredients. Some ingredients are basic supplies that we already own, while others may themselves be recipes. Once...

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LeetCode 3321 - Find X-Sum of All K-Long Subarrays II

LeetCode Problem 3321 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sliding Window, Heap (Priority Queue) Solution Problem Understanding The problem asks us to compute a specialized sum, called the x-sum , over all subarrays of length k in a given integer array nums . Specifically, the x-sum is computed by first counting the occurrences of each element in the subarray, keeping only the top x most frequent elements (with ties...

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LeetCode 1897 - Redistribute Characters to Make All Strings Equal

LeetCode Problem 1897 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks whether it is possible to make all strings in the input array words equal by redistributing characters between them. Specifically, in one operation, you can pick any character from one string and move it to another string at any position. The goal is to determine if, after potentially many such operations, every string...

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LeetCode 2367 - Number of Arithmetic Triplets

LeetCode Problem 2367 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Enumeration Solution Problem Understanding The problem requires us to find the number of arithmetic triplets in a strictly increasing array of integers. An arithmetic triplet (i, j, k) satisfies the conditions i < j < k , nums[j] - nums[i] == diff , and nums[k] - nums[j] == diff . Essentially, we are looking for sequences of three...

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LeetCode 2743 - Count Substrings Without Repeating Character

LeetCode Problem 2743 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of a given string contain only unique characters. A substring is considered special if no character appears more than once inside that substring. We are given a string s containing only lowercase English letters. For every possible contiguous substring, we need to determine whether all characters...

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LeetCode 2791 - Count Paths That Can Form a Palindrome in a Tree

LeetCode Problem 2791 Difficulty: 🔴 Hard Topics: Hash Table, Bit Manipulation, Tree, Depth-First Search Solution I can do that, but the guide will be very long for a single chat response because your required format includes detailed prose, brute force and optimal approaches, proof sketch, Python and Go implementations, worked traces for every example, complexity analysis, comprehensive tests, and edge cases for a Hard problem. To avoid truncation and keep...

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LeetCode 2068 - Check Whether Two Strings are Almost Equivalent

LeetCode Problem 2068 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine whether two strings are "almost equivalent" based on the frequency of each lowercase English letter. We are given two strings, word1 and word2 , both of the same length. For every character from 'a' to 'z' , we compare how many times that character appears in each string. If the...

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LeetCode 3223 - Minimum Length of String After Operations

LeetCode Problem 3223 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to repeatedly remove characters from a string based on a specific rule. Specifically, for any character s[i] in the string, we can remove the nearest occurrence of the same character to the left of i and the nearest occurrence to the right of i . This operation can be applied any number...

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LeetCode 2166 - Design Bitset

LeetCode Problem 2166 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding This problem asks us to design a custom Bitset data structure that supports several operations efficiently. A bitset is simply a sequence of binary values, where each position stores either 0 or 1 . We are given a fixed size during initialization, and initially every bit is set to 0 . After that, the class...

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LeetCode 2561 - Rearranging Fruits

LeetCode Problem 2561 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Sort Solution Problem Understanding This problem gives us two arrays, basket1 and basket2 , where each element represents the cost of a fruit in a basket. Both baskets contain exactly n fruits. Our goal is to make the two baskets equal, where equality means that after sorting both arrays, they contain exactly the same multiset of values. We are...

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LeetCode 2857 - Count Pairs of Points With Distance k

LeetCode Problem 2857 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation Solution LeetCode 2857 - Count Pairs of Points With Distance k Problem Understanding We are given a list of 2D points, where each point is represented as [x, y] , and an integer k . The distance between two points is not the usual Euclidean or Manhattan distance. Instead, it is defined as: $$(x_1 \oplus x_2) + (y_1...

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LeetCode 2830 - Maximize the Profit as the Salesman

LeetCode Problem 2830 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Dynamic Programming, Sorting Solution LeetCode 2830 - Maximize the Profit as the Salesman Problem Understanding This problem gives us n houses arranged on a number line and a collection of purchase offers. Each offer is represented as [start, end, gold] , meaning a buyer wants to purchase every house in the inclusive range [start, end] and is willing...

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LeetCode 2083 - Substrings That Begin and End With the Same Letter

LeetCode Problem 2083 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Counting, Prefix Sum Solution Problem Understanding The problem asks us to count all substrings in a given string s that start and end with the same character. A substring is a contiguous sequence of characters, so the order and adjacency of characters matter. The input s is a non-empty string consisting of lowercase English letters only, and its length...

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LeetCode 2465 - Number of Distinct Averages

LeetCode Problem 2465 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Sorting Solution Problem Understanding The problem requires calculating the number of distinct averages generated from a sequence of numbers using a specific process. The input is an integer array nums of even length. The process repeatedly removes the smallest and largest numbers from the array, calculates their average, and continues until the array is empty. The output is...

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LeetCode 2983 - Palindrome Rearrangement Queries

LeetCode Problem 2983 Difficulty: 🔴 Hard Topics: Hash Table, String, Prefix Sum Solution Problem Understanding We are given a string s of even length n . For every query, we are allowed to independently rearrange two specific substrings: One substring lies completely inside the left half of the string. The other substring lies completely inside the right half of the string. The goal is to determine whether, after rearranging only...

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LeetCode 2325 - Decode the Message

LeetCode Problem 2325 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding This problem is asking us to decode a secret message using a substitution cipher defined by a key string. The key string may include spaces and contains every lowercase letter of the English alphabet at least once. The order in which each letter first appears in the key determines the mapping to the regular alphabet. For example,...

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LeetCode 2540 - Minimum Common Value

LeetCode Problem 2540 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Binary Search Solution Problem Understanding The problem gives us two integer arrays, nums1 and nums2 , and both arrays are already sorted in non-decreasing order. Our task is to find the smallest integer that appears in both arrays. If there is no value shared between the two arrays, we must return -1 . In other words, we need...

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LeetCode 2053 - Kth Distinct String in an Array

LeetCode Problem 2053 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to find the kth distinct string in an array of strings. A string is considered distinct if it appears exactly once in the entire array. The important detail is that the order matters. We are not sorting the strings or rearranging them in any way. Instead, we must consider distinct strings...

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LeetCode 3185 - Count Pairs That Form a Complete Day II

LeetCode Problem 3185 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to count pairs of elements in an integer array hours such that the sum of the two elements is a multiple of 24 , which we call a "complete day." Each pair (i, j) must satisfy i < j . The input is an array of integers, where each integer represents a...

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LeetCode 3039 - Apply Operations to Make String Empty

LeetCode Problem 3039 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting Solution LeetCode 3039 - Apply Operations to Make String Empty Problem Understanding The problem gives us a lowercase string s and defines a repeated operation. During one operation, we scan through all letters from 'a' to 'z' . For each letter, if that letter appears in the current string, we remove its first occurrence. This process continues until...

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LeetCode 2732 - Find a Good Subset of the Matrix

LeetCode Problem 2732 Difficulty: 🔴 Hard Topics: Array, Hash Table, Bit Manipulation, Matrix Solution Problem Understanding The problem asks us to find a subset of rows in a binary matrix where each column’s sum in that subset is at most half of the number of rows chosen, rounded down. In simpler terms, if you pick some rows, no column should have too many 1s compared to the number of rows...

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LeetCode 3137 - Minimum Number of Operations to Make Word K-Periodic

LeetCode Problem 3137 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to transform a given string word of length n into a k-periodic string using the minimum number of operations. A string is k-periodic if it can be formed by repeating a substring s of length k multiple times. Each operation allows us to pick two substrings of length k that start at...

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LeetCode 3297 - Count Substrings That Can Be Rearranged to Contain a String I

LeetCode Problem 3297 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution LeetCode 3297 - Count Substrings That Can Be Rearranged to Contain a String I Problem Understanding We are given two lowercase strings, word1 and word2 . A substring of word1 is considered valid if its characters can be rearranged so that word2 becomes a prefix of the rearranged string. To understand what this means, suppose word2 =...

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LeetCode 2554 - Maximum Number of Integers to Choose From a Range I

LeetCode Problem 2554 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Greedy, Sorting Solution Problem Understanding The problem gives us three inputs: banned , an array of integers that are not allowed to be selected n , which defines the valid integer range [1, n] maxSum , the maximum total sum allowed for all chosen integers We must choose as many distinct integers as possible while satisfying four conditions:...

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LeetCode 2653 - Sliding Subarray Beauty

LeetCode Problem 2653 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution Problem Understanding The problem asks us to compute the beauty of every contiguous subarray of size k within a given integer array nums . The beauty of a subarray is defined as the xth smallest negative number in that subarray. If a subarray contains fewer than x negative numbers, the beauty is 0 . In simpler terms,...

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LeetCode 2748 - Number of Beautiful Pairs

LeetCode Problem 2748 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting, Number Theory Solution LeetCode 2748 - Number of Beautiful Pairs Problem Understanding The problem gives us a 0-indexed integer array nums . We must count how many pairs of indices (i, j) satisfy 0 <= i < j < n and have the following property: Take the first digit of nums[i] . Take the last digit of nums[j]...

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LeetCode 1912 - Design Movie Rental System

LeetCode Problem 1912 Difficulty: 🔴 Hard Topics: Array, Hash Table, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem requires designing a movie rental system for multiple shops, where each shop carries at most one copy of each movie. We are asked to implement a class that efficiently supports searching for available movies, renting them, returning them, and reporting the cheapest currently rented movies. The input consists of...

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LeetCode 2190 - Most Frequent Number Following Key In an Array

LeetCode Problem 2190 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find the integer that appears most frequently immediately after a given key value in the array. More specifically, we are given an integer array nums and an integer key , which is guaranteed to exist in the array. We need to examine every occurrence of key in nums . Whenever we...

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LeetCode 3009 - Maximum Number of Intersections on the Chart

LeetCode Problem 3009 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Binary Indexed Tree, Geometry, Sweep Line, Sorting Solution I can provide the full guide, but I want to avoid giving you a technically incorrect reference document. For LeetCode 3009 , there are two subtly different interpretations that materially change the optimal algorithm and correctness proof: whether intersections at a chart vertex (k, y[k]) count once or twice when both...

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LeetCode 2590 - Design a Todo List

LeetCode Problem 2590 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Sorting Solution Problem Understanding This problem asks us to design a small task management system, similar to a lightweight todo application. We need to implement a TodoList class that supports adding tasks, marking tasks as completed, retrieving all pending tasks for a user, and filtering pending tasks by tag. Each task belongs to a specific user and contains...

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LeetCode 2254 - Design Video Sharing Platform

LeetCode Problem 2254 Difficulty: 🔴 Hard Topics: Hash Table, Design, Heap (Priority Queue) Solution Certainly. Here is a complete, detailed technical solution guide for LeetCode 2254 - Design Video Sharing Platform , following your requested formatting and style. Problem Understanding The problem asks us to design a class to simulate a video sharing platform. Each video is represented as a string of digits, where each digit corresponds to the content...

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LeetCode 3312 - Sorted GCD Pair Queries

LeetCode Problem 3312 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Binary Search, Combinatorics, Counting, Number Theory, Prefix Sum Solution === 1996-G1 === IMO 1996 Shortlist G1 Origin: GBR Problem Let triangle $ABC$ have orthocenter $H$, and let $P$ be a point on its circumcircle, distinct from $A$, $B$, $C$. Let $E$ be the foot of the altitude $BH$, let $PAQB$ and $PARC$ be parallelograms, and let $AQ$ meet $HR$...

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LeetCode 2869 - Minimum Operations to Collect Elements

LeetCode Problem 2869 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation Solution LeetCode 2869 - Minimum Operations to Collect Elements Problem Understanding This problem gives us an array nums and an integer k . We repeatedly perform an operation where we remove the last element of the array and add it to our collection. The goal is to determine the minimum number of such operations required so that our...

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LeetCode 1982 - Find Array Given Subset Sums

LeetCode Problem 1982 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem gives us an integer n and an array sums of length 2^n , which contains every possible subset sum of an unknown array of length n . Our task is to reconstruct any one valid array ans of size n such that the multiset of all subset sums of ans exactly matches the...

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LeetCode 2488 - Count Subarrays With Median K

LeetCode Problem 2488 Difficulty: 🔴 Hard Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding This problem asks us to count how many contiguous subarrays have a median equal to a given value k . The array nums contains every integer from 1 to n exactly once, which means all elements are distinct. That detail is extremely important because it removes ambiguity when comparing values relative to k . The...

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LeetCode 2347 - Best Poker Hand

LeetCode Problem 2347 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem gives us exactly five playing cards. Each card is represented by two pieces of information: ranks[i] represents the numerical rank of the card, from 1 to 13 suits[i] represents the suit of the card, using characters from 'a' to 'd' We need to determine the strongest poker hand that can be formed from these...

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LeetCode 2781 - Length of the Longest Valid Substring

LeetCode Problem 2781 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Sliding Window Solution LeetCode 2781 - Length of the Longest Valid Substring Problem Understanding The problem gives us a string word and a list of forbidden strings forbidden . A substring is considered valid if none of its internal substrings appear in the forbidden list. In other words, for a candidate substring word[l:r] , every contiguous segment inside it...

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LeetCode 2898 - Maximum Linear Stock Score

LeetCode Problem 2898 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution LeetCode 2898 - Maximum Linear Stock Score Problem Understanding We are given a 1-indexed array prices , where prices[i] represents the stock price on day i . We want to choose a subsequence of indices, not necessarily contiguous, such that the selected indices satisfy a special linearity condition. For every pair of consecutive selected indices: $$prices[indexes[j]] - prices[indexes[j-1]] =...

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LeetCode 2421 - Number of Good Paths

LeetCode Problem 2421 Difficulty: 🔴 Hard Topics: Array, Hash Table, Tree, Union-Find, Graph Theory, Sorting Solution Problem Understanding The problem is asking us to count the number of good paths in a tree. A tree is a connected acyclic graph with n nodes and n - 1 edges. Each node has an integer value assigned by the array vals . A good path is defined as a simple path (no...

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LeetCode 2902 - Count of Sub-Multisets With Bounded Sum

LeetCode Problem 2902 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Sliding Window Solution Problem Understanding The problem asks us to count all possible sub-multisets of an array nums such that the sum of elements in each sub-multiset is within a given inclusive range [l, r] . A sub-multiset is like a subset, but it accounts for repeated elements in the original array. That is, each element can appear...

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LeetCode 2451 - Odd String Difference

LeetCode Problem 2451 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution LeetCode 2451 - Odd String Difference Problem Understanding The problem gives us an array of strings where every string has the same length. For each string, we construct a difference array by subtracting the alphabet position of consecutive characters. For example, the string "adc" produces: 'd' - 'a' = 3 'c' - 'd' = -1 So its difference...

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LeetCode 3396 - Minimum Number of Operations to Make Elements in Array Distinct

LeetCode Problem 3396 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem gives us an integer array nums , and our goal is to make all remaining elements distinct. The only allowed operation is removing exactly 3 elements from the beginning of the array. If fewer than 3 elements remain, we remove everything that is left. We need to determine the minimum number of such operations required...

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LeetCode 3144 - Minimum Substring Partition of Equal Character Frequency

LeetCode Problem 3144 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Counting Solution Problem Understanding In this problem, we are given a lowercase English string s , and we must split it into one or more contiguous substrings such that every substring is balanced . A substring is considered balanced when every distinct character inside it appears the same number of times. The actual frequency does not matter, only...

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LeetCode 2845 - Count of Interesting Subarrays

LeetCode Problem 2845 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem is asking us to count the number of interesting subarrays in a given array nums . An interesting subarray is defined by a modular counting condition: within any subarray nums[l..r] , count the number of elements nums[i] where nums[i] % modulo == k . If this count, cnt , also satisfies cnt %...

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LeetCode 2729 - Check if The Number is Fascinating

LeetCode Problem 2729 Difficulty: 🟢 Easy Topics: Hash Table, Math Solution Problem Understanding The problem gives us a three digit integer n . We must determine whether n is a fascinating number. A number is considered fascinating when we concatenate three values together: n 2 * n 3 * n After concatenation, the resulting number must contain every digit from 1 through 9 exactly once. The digit 0 must not...

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LeetCode 3128 - Right Triangles

LeetCode Problem 3128 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Combinatorics, Counting Solution Problem Understanding The problem gives us a binary matrix grid , where each cell contains either 0 or 1 . We need to count how many valid right triangles can be formed using cells whose value is 1 . A valid right triangle in this problem is defined by selecting exactly three cells containing 1 such...

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LeetCode 2718 - Sum of Matrix After Queries

LeetCode Problem 2718 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution LeetCode 2718 - Sum of Matrix After Queries Problem Understanding The problem gives us an integer n representing the size of an n x n matrix. Initially, every cell in the matrix contains 0 . We are also given a list of queries. Each query has the form: [type, index, value] There are two kinds of operations: If type...

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LeetCode 1865 - Finding Pairs With a Certain Sum

LeetCode Problem 1865 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design Solution Problem Understanding This problem asks us to design a mutable data structure that supports two operations efficiently over two arrays, nums1 and nums2 . The first array, nums1 , is fixed after initialization and never changes. The second array, nums2 , is mutable, meaning its elements can be updated through add(index, val) operations. The goal of the data...

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LeetCode 3295 - Report Spam Message

LeetCode Problem 3295 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution LeetCode 3295 - Report Spam Message Problem Understanding This problem asks us to determine whether a given message should be classified as spam. We are given two arrays of strings: message , which contains the words appearing in the message. bannedWords , which contains words that are considered banned. A message is considered spam if at least two...

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LeetCode 2716 - Minimize String Length

LeetCode Problem 2716 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to minimize the length of a given string s by repeatedly performing two types of deletion operations. In the first operation, we can pick a character at some index and delete the closest identical character to its left if one exists. In the second operation, we can pick a character and delete the...

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LeetCode 3325 - Count Substrings With K-Frequency Characters I

LeetCode Problem 3325 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count all substrings of a given string s such that at least one character in the substring appears at least k times . In simpler terms, for every substring we consider, we check whether there is any character that has frequency greater than or equal to k . If there...

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LeetCode 3164 - Find the Number of Good Pairs II

LeetCode Problem 3164 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding We are given two integer arrays, nums1 and nums2 , along with a positive integer k . A pair of indices (i, j) is considered good if: $$nums1[i] \bmod (nums2[j] \times k) = 0$$ In other words, nums1[i] must be divisible by nums2[j] * k . The task is to count how many such index pairs exist....

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LeetCode 2453 - Destroy Sequential Targets

LeetCode Problem 2453 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem presents a scenario where we have a list of positive integers nums representing targets placed on a number line. We also have an integer space representing the step interval of a machine. The machine can destroy all targets starting from an initial seed value nums[i] in steps of space . In other words, if...

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LeetCode 2268 - Minimum Number of Keypresses

LeetCode Problem 2268 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Counting Solution Problem Understanding The problem asks us to determine the minimum number of keypresses required to type a given string s using a keypad with 9 buttons. Each button can map to at most 3 letters , and each letter must be mapped to exactly one button. When typing a character, the number of presses depends on...

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LeetCode 2516 - Take K of Each Character From Left and Right

LeetCode Problem 2516 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to determine the minimum number of characters to take from either end of a string s consisting only of the letters 'a' , 'b' , and 'c' so that we collect at least k of each character . You are allowed to take characters from the leftmost or rightmost position of...

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LeetCode 2475 - Number of Unequal Triplets in Array

LeetCode Problem 2475 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem gives us a 0-indexed integer array nums , and we must count how many triplets (i, j, k) satisfy two conditions. First, the indices must be strictly increasing: 0 <= i < j < k < nums.length Second, the values at those indices must all be different from each other: nums[i] != nums[j] nums[i]...

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LeetCode 2186 - Minimum Number of Steps to Make Two Strings Anagram II

LeetCode Problem 2186 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to find the minimum number of steps required to make two strings, s and t , anagrams of each other. An anagram means that both strings must contain the exact same characters with the same frequency, though their order can differ. In this problem, a "step" allows appending any character to either...

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LeetCode 3177 - Find the Maximum Length of a Good Subsequence II

LeetCode Problem 3177 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming Solution LeetCode 3177 - Find the Maximum Length of a Good Subsequence II Problem Understanding We are given an integer array nums and a non-negative integer k . We must find the maximum possible length of a subsequence such that the number of adjacent unequal pairs is at most k . A subsequence is formed by deleting zero...

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LeetCode 1832 - Check if the Sentence Is Pangram

LeetCode Problem 1832 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding This problem asks us to determine whether a given string is a pangram. A pangram is a sentence that contains every lowercase English letter, from 'a' through 'z' , at least once. The input is a single string named sentence . The problem guarantees that the string contains only lowercase English letters, which simplifies the implementation because...

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LeetCode 3016 - Minimum Number of Pushes to Type Word II

LeetCode Problem 3016 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Counting Solution LeetCode 3016 - Minimum Number of Pushes to Type Word II Problem Understanding This problem asks us to redesign a telephone keypad so that typing a given word requires the fewest total key presses possible. A traditional telephone keypad contains keys 2 through 9 , giving us exactly 8 available keys . Each lowercase English letter...

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LeetCode 2615 - Sum of Distances

LeetCode Problem 2615 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to compute, for every index i in the array nums , the total distance between i and every other index j where nums[j] == nums[i] . More formally, for each position i , we need to calculate: $$\sum |i - j|$$ for all indices j such that: nums[j] == nums[i] j...

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LeetCode 2956 - Find Common Elements Between Two Arrays

LeetCode Problem 2956 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding This problem asks us to compare two integer arrays and count how many elements from one array appear in the other. More specifically, we need to compute two values: answer1 is the number of indices i in nums1 such that nums1[i] appears at least once in nums2 . answer2 is the number of indices i in nums2...

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LeetCode 2150 - Find All Lonely Numbers in the Array

LeetCode Problem 2150 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution LeetCode 2147 - Number of Ways to Divide a Long Corridor Problem Understanding The corridor is represented as a string where each character is either 'S' for a seat or 'P' for a plant. We already have fixed dividers at both ends of the corridor, and we may optionally place additional dividers between adjacent positions. The goal is...

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LeetCode 2982 - Find Longest Special Substring That Occurs Thrice II

LeetCode Problem 2982 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Sliding Window, Counting Solution LeetCode 2982 - Find Longest Special Substring That Occurs Thrice II Problem Understanding The problem asks us to find the maximum length of a special substring that appears at least three times in the given string. A substring is considered special if it consists entirely of a single repeated character. Examples include: "a" "bbb"...

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LeetCode 2711 - Difference of Number of Distinct Values on Diagonals

LeetCode Problem 2711 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution Problem Understanding The problem is asking us to compute a new matrix answer based on a given m x n grid, where each cell in answer represents the absolute difference between the number of distinct elements on the diagonal above and to the left of the current cell, and the number of distinct elements on the diagonal below...

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LeetCode 2521 - Distinct Prime Factors of Product of Array

LeetCode Problem 2521 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Number Theory Solution Problem Understanding The problem asks us to compute the number of distinct prime factors in the product of an array of positive integers, nums . In simpler terms, we are given a list of numbers, and we want to know how many unique prime numbers appear if we were to multiply all the numbers together and...

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LeetCode 2815 - Max Pair Sum in an Array

LeetCode Problem 2815 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 2815 - Max Pair Sum in an Array Problem Understanding The problem gives us an array of integers nums . For every number, we can determine its largest digit . For example: 2536 has digits 2, 5, 3, 6 , so its largest digit is 6 112 has digits 1, 1, 2 , so its largest digit is...

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LeetCode 3170 - Lexicographically Minimum String After Removing Stars

LeetCode Problem 3170 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Greedy, Heap (Priority Queue) Solution Problem Understanding The problem asks us to transform a string s that may contain the '*' character into a lexicographically smallest string by repeatedly applying a deletion operation. Specifically, for each '*' in the string, we must remove it along with the smallest non- '*' character to its left . If there are multiple...

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LeetCode 3265 - Count Almost Equal Pairs I

LeetCode Problem 3265 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting, Enumeration Solution Problem Understanding The problem asks us to find the number of index pairs (i, j) in an array nums such that i < j and the integers nums[i] and nums[j] are almost equal . Two integers are considered almost equal if you can make them equal by swapping any two digits at most once in either...

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LeetCode 2949 - Count Beautiful Substrings II

LeetCode Problem 2949 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Number Theory, Prefix Sum Solution Problem Understanding The problem asks us to find the number of non-empty substrings in a given string s that satisfy two conditions simultaneously. First, the substring must have an equal number of vowels and consonants. Second, the product of the number of vowels and consonants must be divisible by a given integer k ....

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LeetCode 2531 - Make Number of Distinct Characters Equal

LeetCode Problem 2531 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Exploration The problem is a pure inequality with a geometric constraint. The condition that $\angle BDC=\frac{\pi}{2}$ gives $$BC^2=BD^2+CD^2$$ by the Pythagorean theorem in $\triangle BDC$. The second condition, that the foot $H$ of the perpendicular from $D$ to plane $ABC$ is the orthocenter of $\triangle ABC$, is much more restrictive than it first appears. It suggests introducing coordinates...

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LeetCode 2080 - Range Frequency Queries

LeetCode Problem 2080 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design, Segment Tree Solution Problem Understanding The problem asks us to design a data structure that can efficiently answer frequency queries on subarrays. We are given an integer array arr , and we need to support repeated queries of the form: Given left , right , and value Return how many times value appears in the subarray arr[left...right]...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2250 - Count Number of Rectangles Containing Each Point

LeetCode Problem 2250 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Binary Indexed Tree, Sorting Solution Here is the complete, detailed technical solution guide for LeetCode 2250 - Count Number of Rectangles Containing Each Point in a single, comprehensive response. Problem Understanding The problem provides two arrays: rectangles and points . Each element of rectangles is [li, hi] , representing a rectangle with its bottom-left corner at (0, 0)...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1311 - Get Watched Videos by Your Friends

LeetCode Problem 1311 Difficulty: 🟡 Medium Topics: Array, Hash Table, Breadth-First Search, Graph Theory, Sorting Solution Problem Understanding This problem models a social network as an undirected graph. Each person is represented by an integer ID from 0 to n - 1 . The friends array describes the graph connections, while watchedVideos[i] contains the list of videos watched by person i . The goal is to find all videos watched...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 705 - Design HashSet

LeetCode Problem 705 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Design, Hash Function Solution Problem Understanding The problem asks us to design our own HashSet implementation without using any built in hash table libraries such as Python's set or Go's built in map type as the primary solution idea. A HashSet is a data structure that stores unique values. It supports three operations: add(key) , insert a value...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 873 - Length of Longest Fibonacci Subsequence

LeetCode Problem 873 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming Solution Problem Understanding The problem asks us to find the length of the longest subsequence in a strictly increasing array that forms a Fibonacci-like sequence. A Fibonacci-like sequence follows the rule: $x_i + x_{i+1} = x_{i+2}$ for every valid index in the sequence, and the sequence must contain at least three numbers. Unlike the classic Fibonacci sequence, the...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1357 - Apply Discount Every n Orders

LeetCode Problem 1357 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design Solution Problem Understanding The problem describes a supermarket that sells products identified by unique integer IDs. Each product has a corresponding price. Customers purchase products in certain amounts, generating a bill that is the sum of the prices multiplied by the amounts purchased. The supermarket offers a periodic discount: every nth customer gets a percentage discount off their subtotal....

Practice › LeetCode › LeetCode 08xx ›

LeetCode 815 - Bus Routes

LeetCode Problem 815 Difficulty: 🔴 Hard Topics: Array, Hash Table, Breadth-First Search Solution Problem Understanding This problem asks us to determine the minimum number of buses required to travel from a given starting bus stop ( source ) to a destination bus stop ( target ) given a set of bus routes. Each route is represented as a list of bus stops that a particular bus cycles through indefinitely. For...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 246 - Strobogrammatic Number

LeetCode Problem 246 Difficulty: 🟢 Easy Topics: Hash Table, Two Pointers, String Solution Problem Understanding The problem asks us to determine whether a given numeric string remains valid and unchanged when rotated 180 degrees. This property is called being "strobogrammatic". A strobogrammatic number is formed only by digits that still represent valid digits after rotation. The valid transformations are: 0 -> 0 1 -> 1 6 -> 9 8 ->...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1638 - Count Substrings That Differ by One Character

LeetCode Problem 1638 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Enumeration Solution Problem Understanding The problem asks us to count how many pairs of substrings, one taken from s and one taken from t , differ by exactly one character. A substring is contiguous, so for a string like "abcde" , valid substrings include "a" , "bc" , "abcd" , and "cde" . We are not allowed to...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 974 - Subarray Sums Divisible by K

LeetCode Problem 974 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to count how many non-empty contiguous subarrays have a sum that is divisible by a given integer k . A subarray is any continuous segment of the array. For example, in the array [1,2,3] , the subarrays include [1] , [2] , [3] , [1,2] , [2,3] , and [1,2,3] ....

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1487 - Making File Names Unique

LeetCode Problem 1487 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to simulate how a file system assigns folder names when duplicate names appear. We are given an array names , where names[i] represents the folder name requested at the i-th minute. The file system must ensure that every created folder has a unique name. If a requested name has never been used...

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LeetCode 1496 - Path Crossing

LeetCode Problem 1496 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding This problem asks us to simulate movement on a 2D plane according to a string of directional instructions. Each character in the string represents a move of one unit in a cardinal direction: 'N' for north (up), 'S' for south (down), 'E' for east (right), and 'W' for west (left). You always start at the origin (0,...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 457 - Circular Array Loop

LeetCode Problem 457 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers Solution Problem Understanding This problem asks us to determine whether a circular array contains a valid cycle under a specific movement rule. Each element in the array represents how far we move from the current index. Positive numbers move forward, negative numbers move backward, and movement wraps around because the array is circular. The key requirement is that...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2067 - Number of Equal Count Substrings

LeetCode Problem 2067 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window, Counting Solution Problem Understanding The problem asks us to count the number of substrings in a string s such that every unique character in the substring occurs exactly count times . A substring is a contiguous segment of the string, so we are only considering consecutive characters. Inputs are: s : a string of lowercase English letters, length...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 923 - 3Sum With Multiplicity

LeetCode Problem 923 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Sorting, Counting Solution Problem Understanding The problem asks us to count how many index triplets (i, j, k) satisfy two conditions: The indices must follow the order i < j < k The values at those indices must sum to the given target Formally, we want: arr[i] + arr[j] + arr[k] == target The important detail is that...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 291 - Word Pattern II

LeetCode Problem 291 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution Problem Understanding The problem asks us to determine whether a given string s can be formed by substituting each character in a pattern with a non-empty string , under a bijective mapping . Bijective means that every character maps to exactly one unique substring and no two characters map to the same substring. Essentially, each character in the...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 817 - Linked List Components

LeetCode Problem 817 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List Solution Problem Understanding The problem gives us a singly linked list where every node contains a unique integer value. We are also given an array nums , and every value in nums is guaranteed to appear somewhere in the linked list. We need to count how many connected components exist among the nodes whose values belong to nums...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2001 - Number of Pairs of Interchangeable Rectangles

LeetCode Problem 2001 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting, Number Theory Solution LeetCode 2001 - Number of Pairs of Interchangeable Rectangles Problem Understanding This problem asks us to count how many pairs of rectangles are interchangeable. Each rectangle is represented by two integers, width and height . Two rectangles are considered interchangeable if their width-to-height ratios are exactly the same. In mathematical terms, rectangles i and j...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 454 - 4Sum II

LeetCode Problem 454 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem gives four integer arrays, nums1 , nums2 , nums3 , and nums4 , each containing n elements. We must count how many index tuples (i, j, k, l) satisfy: nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0 The important detail is that we are counting tuples of indices, not unique value combinations. If the...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1224 - Maximum Equal Frequency

LeetCode Problem 1224 Difficulty: 🔴 Hard Topics: Array, Hash Table Solution Problem Understanding The problem gives an array nums consisting of positive integers. We must find the longest prefix of the array such that, after removing exactly one element from that prefix, every remaining number appears the same number of times. A prefix means the subarray starting from index 0 and ending at some index i . We are free...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1002 - Find Common Characters

LeetCode Problem 1002 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us an array of lowercase strings called words . We must return all characters that appear in every string in the array, including duplicate occurrences. The important detail is that duplicates matter. If a character appears multiple times in every word, then it must appear multiple times in the answer as well. For...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 229 - Majority Element II

LeetCode Problem 229 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem asks us to find every element in an integer array that appears more than ⌊ n/3 ⌋ times, where n is the length of the array. The floor notation means we round down to the nearest integer. For example, if the array length is 8, then ⌊ 8/3 ⌋ = 2 , so...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1400 - Construct K Palindrome Strings

LeetCode Problem 1400 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Counting Solution Problem Understanding The problem is asking whether a given string s can be rearranged to form exactly k non-empty palindrome strings using all characters in s . In other words, we need to partition s into k palindromes such that each character of s is used exactly once. A palindrome is a string that reads the same...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1365 - How Many Numbers Are Smaller Than the Current Number

LeetCode Problem 1365 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Counting Sort Solution Problem Understanding The problem asks us to determine, for each element in an array, how many elements in the same array are strictly smaller than it. In other words, for an element nums[i] , we count all elements nums[j] such that nums[j] < nums[i] and j != i . The input is a list of integers...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 681 - Next Closest Time

LeetCode Problem 681 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking, Enumeration Solution Problem Understanding The problem gives us a valid time string in the format "HH:MM" and asks us to construct the next chronological time using only the digits already present in the original time. The important detail is that digits may be reused any number of times. If the input is "19:34" , the allowed digits are {1,...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 451 - Sort Characters By Frequency

LeetCode Problem 451 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting, Heap (Priority Queue), Bucket Sort, Counting Solution Problem Understanding The problem asks us to rearrange the characters of a string so that characters with higher frequency appear before characters with lower frequency. The output string must group identical characters together, and those groups must appear in descending order of occurrence count. For example, if the input is "tree" ,...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 653 - Two Sum IV - Input is a BST

LeetCode Problem 653 Difficulty: 🟢 Easy Topics: Hash Table, Two Pointers, Tree, Depth-First Search, Breadth-First Search, Binary Search Tree, Binary Tree Solution LeetCode 653, Two Sum IV - Input is a BST Problem Understanding This problem gives us the root of a Binary Search Tree (BST) and an integer k . We must determine whether there are two distinct nodes in the tree whose values add up to k ....

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1713 - Minimum Operations to Make a Subsequence

LeetCode Problem 1713 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Greedy Solution LeetCode 1713, Minimum Operations to Make a Subsequence Problem Understanding The problem gives us two arrays, target and arr . The target array contains distinct integers, which is extremely important. The arr array may contain duplicates. We are allowed to perform insert operations on arr . In a single operation, we can insert any integer at...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 763 - Partition Labels

LeetCode Problem 763 Difficulty: 🟡 Medium Topics: Hash Table, Two Pointers, String, Greedy Solution Problem Understanding The problem is asking us to partition a string s into as many contiguous parts as possible such that no letter appears in more than one part. In other words, each character in the string should be confined to a single segment. After partitioning, concatenating all the segments in order should reproduce the original...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1396 - Design Underground System

LeetCode Problem 1396 Difficulty: 🟡 Medium Topics: Hash Table, String, Design Solution Problem Understanding This problem asks us to design a small tracking system for an underground railway network. The system must support three operations: Recording when a customer checks into a station. Recording when a customer checks out from a station. Calculating the average travel time between two stations. Each customer is identified by a unique integer id ....

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1794 - Count Pairs of Equal Substrings With Minimum Difference

LeetCode Problem 1794 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy Solution Problem Understanding This problem asks us to count quadruples of indices (i, j, a, b) where substrings from two given strings are equal and the difference j - a is minimized. Specifically, i and j define a substring in firstString , while a and b define a substring in secondString . The substrings themselves must be identical, and...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 692 - Top K Frequent Words

LeetCode Problem 692 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, Sorting, Heap (Priority Queue), Bucket Sort, Counting Solution Problem Understanding The problem gives us an array of strings called words and an integer k . Each string represents a word, and words may appear multiple times in the array. Our task is to return the k most frequent words. The result must follow two ordering rules. First, words...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 567 - Permutation in String

LeetCode Problem 567 Difficulty: 🟡 Medium Topics: Hash Table, Two Pointers, String, Sliding Window Solution Problem Understanding This problem asks us to determine whether any permutation of s1 appears as a contiguous substring inside s2 . A permutation means the characters are rearranged, but the frequency of each character remains the same. For example, the permutations of "ab" are "ab" and "ba" . If either of these appears somewhere inside...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 264 - Ugly Number II

LeetCode Problem 264 Difficulty: 🟡 Medium Topics: Hash Table, Math, Dynamic Programming, Heap (Priority Queue) Solution Problem Understanding The problem asks us to generate the sequence of ugly numbers and return the n th value in that sequence. An ugly number is defined as a positive integer whose only prime factors are 2 , 3 , and 5 . This means every ugly number can be formed by multiplying previous...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1807 - Evaluate the Bracket Pairs of a String

LeetCode Problem 1807 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to process a string s that contains bracketed keys like (name) or (age) and replace them with corresponding values provided in the knowledge array. Each element of knowledge is a two-element array [key, value] mapping a key to its replacement value. If a key in s is not present in knowledge ,...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 659 - Split Array into Consecutive Subsequences

LeetCode Problem 659 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Heap (Priority Queue) Solution Problem Understanding The problem gives us a sorted integer array nums , and asks whether we can divide every element into one or more subsequences that satisfy two conditions. First, each subsequence must consist of consecutive increasing integers. This means every next value must be exactly 1 larger than the previous value. For example, [3,4,5]...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1202 - Smallest String With Swaps

LeetCode Problem 1202 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find, Sorting Solution Problem Understanding The problem gives us a string s and a list of index pairs called pairs . Each pair [a, b] means we are allowed to swap the characters at positions a and b . The important detail is that swaps can be performed any number of times. This changes the...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 632 - Smallest Range Covering Elements from K Lists

LeetCode Problem 632 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Sliding Window, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem gives us k sorted integer lists, and we must find the smallest inclusive range [a, b] such that the range contains at least one element from every list. Each list is already sorted in non-decreasing order, which is a very important property. Instead of considering arbitrary combinations of...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 140 - Word Break II

LeetCode Problem 140 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Dynamic Programming, Backtracking, Trie, Memoization Solution Problem Understanding The problem asks us to segment a given string s into all possible sentences where each word in the sentence exists in a given dictionary wordDict . Essentially, we need to find all ways to insert spaces into the string such that every resulting substring is a valid dictionary word. The...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 916 - Word Subsets

LeetCode Problem 916 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to determine which strings in words1 satisfy all character requirements imposed by every string in words2 . A string b is considered a subset of another string a if every character in b appears in a at least as many times as it appears in b . This includes repeated characters. For...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1679 - Max Number of K-Sum Pairs

LeetCode Problem 1679 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Sorting Solution Problem Understanding The problem asks us to repeatedly remove pairs of numbers from an array such that the sum of each selected pair equals a target value k . Every time we successfully remove one valid pair, it counts as one operation. Our goal is to determine the maximum number of operations that can be performed...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 736 - Parse Lisp Expression

LeetCode Problem 736 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Recursion Solution Problem Understanding The problem asks us to evaluate a Lisp-like expression represented as a string. The expression can contain integers, variables, and three special operations: let , add , and mult . An expression evaluates to a single integer value. The challenge is not arithmetic itself, but correctly handling nested scopes and variable shadowing. A let expression...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1016 - Binary String With Substrings Representing 1 To N

LeetCode Problem 1016 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Sliding Window Solution Problem Understanding The problem asks us to determine whether a given binary string s contains all binary representations of integers from 1 to n as substrings . In other words, for every integer i such that 1 <= i <= n , if you convert i to its binary form, that binary string must appear...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1726 - Tuple with Same Product

LeetCode Problem 1726 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to count all tuples (a, b, c, d) from a given array of distinct positive integers such that the product of the first two elements equals the product of the second two elements, a * b = c * d , and all elements are distinct. Importantly, permutations of the same numbers...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 460 - LFU Cache

LeetCode Problem 460 Difficulty: 🔴 Hard Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design a cache that supports two operations: get(key) returns the value associated with a key if it exists, otherwise -1 put(key, value) inserts or updates a key-value pair Unlike a normal cache, eviction is not based only on recency. Instead, this cache uses the Least Frequently Used policy....

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1086 - High Five

LeetCode Problem 1086 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem asks us to compute the top five average score for each student given a list of [ID, score] pairs. Each ID represents a unique student, and score represents a single score they received. The goal is to return a list of [ID, topFiveAverage] pairs, where topFiveAverage is calculated by taking the...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1481 - Least Number of Unique Integers after K Removals

LeetCode Problem 1481 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Counting Solution Problem Understanding This problem asks us to minimize the number of unique integers in an array after removing exactly k elements. In other words, we are given a list arr and a number k , and we need to strategically remove k elements so that the count of distinct integers left in the array is as...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 954 - Array of Doubled Pairs

LeetCode Problem 954 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting Solution Problem Understanding The problem asks whether it is possible to reorder an even-length array arr of integers such that every element can be paired with another element that is exactly double its value. In other words, after reordering, for every index i in the first half of the array, arr[2 * i + 1] should equal 2...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1255 - Maximum Score Words Formed by Letters

LeetCode Problem 1255 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Dynamic Programming, Backtracking, Bit Manipulation, Counting, Bitmask Solution Problem Understanding This problem asks us to select a subset of words that can be constructed using a limited supply of letters, such that the total score of all chosen words is maximized. We are given three inputs: words , an array of strings letters , a multiset of available characters...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3043 - Find the Length of the Longest Common Prefix

LeetCode Problem 3043 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding We are given two arrays of positive integers , arr1 and arr2 . The task is to determine the maximum length of a common digit prefix between any pair of numbers (x, y) where x comes from arr1 and y comes from arr2 . A prefix of a number means taking digits from the beginning,...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1370 - Increasing Decreasing String

LeetCode Problem 1370 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem requires us to reorder a string s by repeatedly picking characters in a specific increasing and decreasing pattern. In other words, we need to simulate a process where, at each iteration, we first pick the smallest character and keep picking increasingly larger characters until we cannot pick anymore. Then, we switch directions: we pick...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 217 - Contains Duplicate

LeetCode Problem 217 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem gives an integer array nums and asks whether any number appears more than once. If at least one value is repeated, we return true . If every value appears exactly once, we return false . The input is a list of integers. The output is a boolean value that answers a simple question: does...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 697 - Degree of an Array

LeetCode Problem 697 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to find the shortest contiguous subarray of an array nums that has the same degree as the original array. The degree of an array is defined as the maximum frequency of any element in that array. In other words, if the element that appears the most appears k times, then k is the...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 963 - Minimum Area Rectangle II

LeetCode Problem 963 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Geometry Solution Problem Understanding The problem gives us a collection of unique points on a 2D plane. Our task is to determine the minimum possible area of any rectangle that can be formed using exactly four of these points as vertices. The important detail is that the rectangle does not need to be aligned with the coordinate axes. This...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1248 - Count Number of Nice Subarrays

LeetCode Problem 1248 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Sliding Window, Prefix Sum Solution Problem Understanding The problem asks us to count how many contiguous subarrays contain exactly k odd numbers. We are given an integer array nums and an integer k . A subarray is any continuous segment of the array, meaning the elements must appear next to each other in the original order. A subarray is...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1165 - Single-Row Keyboard

LeetCode Problem 1165 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem describes a single-row keyboard where each key is positioned at a unique index from 0 to 25. You are given a string keyboard of length 26 that specifies the layout of all lowercase English letters. The initial finger position is at index 0, and moving the finger from index i to index j takes |i...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1172 - Dinner Plate Stacks

LeetCode Problem 1172 Difficulty: 🔴 Hard Topics: Hash Table, Stack, Design, Heap (Priority Queue) Solution Problem Understanding The problem asks us to design a data structure that behaves like an infinite row of stacks, where every stack has the same fixed capacity. Instead of working with a single stack, we maintain many stacks indexed from left to right starting at 0 . The operations are constrained by specific rules: When...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 12 - Integer to Roman

LeetCode Problem 12 Difficulty: 🟡 Medium Topics: Hash Table, Math, String Solution Problem Understanding The problem asks us to convert a positive integer into its Roman numeral representation. Roman numerals use combinations of specific symbols to represent values, and the rules for combining those symbols are strict. Unlike a positional number system such as decimal, Roman numerals are constructed greedily from the largest possible symbol values down to the smallest....

Practice › LeetCode › LeetCode 09xx ›

LeetCode 959 - Regions Cut By Slashes

LeetCode Problem 959 Difficulty: 🟡 Medium Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Matrix Solution Let's dive into a full, detailed technical solution guide for LeetCode 959 - Regions Cut By Slashes following your formatting rules. Problem Understanding The problem gives us an n x n grid where each cell contains either a forward slash '/' , a backslash '\\' , or a blank space ' ' ....

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LeetCode 1178 - Number of Valid Words for Each Puzzle

LeetCode Problem 1178 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Bit Manipulation, Trie Solution Problem Understanding The problem asks us to determine, for each given puzzle string, how many words from a given list words are valid for that puzzle. A word is valid for a puzzle if it contains the first character of the puzzle and all characters of the word are also present in the puzzle. The...

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LeetCode 76 - Minimum Window Substring

LeetCode Problem 76 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find the smallest contiguous substring inside string s that contains every character from string t , including duplicate occurrences. A substring must consist of consecutive characters. We are not allowed to reorder characters or skip positions. The window must contain every required character with at least the same frequency as...

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LeetCode 1490 - Clone N-ary Tree

LeetCode Problem 1490 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search Solution LeetCode 1490 - Clone N-ary Tree Problem Understanding This problem asks us to create a deep copy of an N-ary tree. An N-ary tree is a tree where each node can have zero or more children, unlike a binary tree which limits each node to at most two children. Each node contains two fields: val...

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LeetCode 865 - Smallest Subtree with all the Deepest Nodes

LeetCode Problem 865 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding The problem gives us the root of a binary tree and asks us to find the smallest subtree that contains all of the deepest nodes in the tree. The depth of a node is defined as the number of edges between that node and the root. The root itself has depth...

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LeetCode 1347 - Minimum Number of Steps to Make Two Strings Anagram

LeetCode Problem 1347 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks us to determine the minimum number of character replacements needed to transform string t into an anagram of string s . Two strings are anagrams if they contain exactly the same characters with the same frequencies, regardless of order. Since both strings are guaranteed to have the same length, we never need to...

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LeetCode 389 - Find the Difference

LeetCode Problem 389 Difficulty: 🟢 Easy Topics: Hash Table, String, Bit Manipulation, Sorting Solution Problem Understanding This problem gives us two strings, s and t . The string t is created by taking all characters from s , shuffling them into a different order, and then inserting exactly one additional character somewhere in the string. Our task is to identify and return that extra character. In other words, every character...

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LeetCode 929 - Unique Email Addresses

LeetCode Problem 929 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a list of email addresses and asks how many unique destinations actually receive emails after applying Gmail-like normalization rules. Each email consists of two parts separated by the '@' symbol: The local name, which appears before '@' The domain name, which appears after '@' The domain name is always preserved exactly as...

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LeetCode 421 - Maximum XOR of Two Numbers in an Array

LeetCode Problem 421 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation, Trie Solution LeetCode 421 - Maximum XOR of Two Numbers in an Array Problem Understanding The problem gives us an array of non-negative integers called nums . We must choose two indices i and j such that 0 <= i <= j < n , then compute: nums[i] XOR nums[j] The goal is to return the largest XOR...

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LeetCode 448 - Find All Numbers Disappeared in an Array

LeetCode Problem 448 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to find all the integers in the range [1, n] that are missing from an input array nums of length n . Each element in nums is guaranteed to be an integer between 1 and n inclusive, but some numbers in this range may not appear in the array, and some numbers may...

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LeetCode 737 - Sentence Similarity II

LeetCode Problem 737 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find Solution Problem Understanding This problem asks us to determine whether two sentences are considered similar based on a set of similarity relationships between words. Each sentence is represented as an array of strings, where each string is a single word. We are also given a list of word pairs, where each pair indicates that...

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LeetCode 548 - Split Array with Equal Sum

LeetCode Problem 548 Difficulty: 🔴 Hard Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to determine whether an integer array nums can be split into four non-empty subarrays with equal sums by selecting three indices i , j , and k that satisfy strict ordering constraints: 0 < i , i + 1 < j , j + 1 < k < n - 1...

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LeetCode 752 - Open the Lock

LeetCode Problem 752 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Breadth-First Search Solution Problem Understanding The problem presents a lock with four wheels, each wheel labeled from '0' to '9' , which can rotate forwards or backwards. The lock starts at '0000' , and we are given a list of "deadends," which are lock states that will cause the lock to freeze if reached. The goal is to determine...

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LeetCode 819 - Most Common Word

LeetCode Problem 819 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to process a paragraph of text and determine which word appears most frequently, while ignoring a given list of banned words. The final answer must be returned in lowercase. Several important details make this problem slightly more involved than simple word counting. First, the paragraph may contain punctuation such as commas,...

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LeetCode 37 - Sudoku Solver

LeetCode Problem 37 Difficulty: 🔴 Hard Topics: Array, Hash Table, Backtracking, Matrix Solution Problem Understanding The problem asks us to build a complete Sudoku solver. We are given a partially filled 9 x 9 grid where each cell contains either a digit from '1' to '9' or the character '.' , which represents an empty space. Our goal is to fill every empty cell so that the final board satisfies...

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LeetCode 677 - Map Sum Pairs

LeetCode Problem 677 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design a custom data structure that behaves like a map from strings to integers, while also supporting efficient prefix-based sum queries. There are two operations: insert(key, val) This operation stores a string key with an integer value. If the key already exists, its old value must be replaced with the...

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LeetCode 1817 - Finding the Users Active Minutes

LeetCode Problem 1817 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding This problem asks us to analyze a collection of user activity logs and determine how many users have a specific number of unique active minutes. Each log entry is represented as: [IDi, timei] where: IDi is the user's ID timei is the minute when that user performed an action A user may perform multiple actions during the...

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LeetCode 1485 - Clone Binary Tree With Random Pointer

LeetCode Problem 1485 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution That’s a detailed reference guide with multiple required sections and full implementations. Before I start, please confirm one detail: Do you want the complete guide for LeetCode 1485 in a single response , including all sections (Problem Understanding, Approaches, Algorithm Walkthrough, Python Solution, Go Solution, Worked Examples, Complexity Analysis, Test Cases, and Edge...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 49 - Group Anagrams

LeetCode Problem 49 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem asks us to group together all strings that are anagrams of each other. Two strings are considered anagrams if they contain exactly the same characters with the same frequencies, but possibly in a different order. For example, "eat" , "tea" , and "ate" are all anagrams because each string contains one 'a' ,...

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LeetCode 508 - Most Frequent Subtree Sum

LeetCode Problem 508 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem asks us to compute the subtree sum for every node in a binary tree, then determine which subtree sum appears most frequently. A subtree rooted at a node includes that node and all of its descendants. The subtree sum is simply the sum of every value contained in that subtree. For...

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LeetCode 748 - Shortest Completing Word

LeetCode Problem 748 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a string called licensePlate and an array of candidate words called words . We must find the shortest word that satisfies all the letter requirements contained in licensePlate . The key detail is that only alphabetic characters matter. Digits and spaces should be ignored completely. In addition, uppercase and lowercase letters are...

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LeetCode 126 - Word Ladder II

LeetCode Problem 126 Difficulty: 🔴 Hard Topics: Hash Table, String, Backtracking, Breadth-First Search Solution Problem Understanding This problem asks us to find all shortest transformation sequences from beginWord to endWord , where each transformation changes exactly one character and every intermediate word must exist in wordList . We can think of the problem as navigating through a graph. Each word is a node, and two words are connected if they...

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LeetCode 982 - Triples with Bitwise AND Equal To Zero

LeetCode Problem 982 Difficulty: 🔴 Hard Topics: Array, Hash Table, Bit Manipulation Solution Problem Understanding This problem asks us to count the number of triples (i, j, k) from an integer array nums such that the bitwise AND of the three numbers at these indices equals zero. In other words, we want all combinations where nums[i] & nums[j] & nums[k] == 0 . The input is an array of integers...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1348 - Tweet Counts Per Frequency

LeetCode Problem 1348 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Design, Sorting, Ordered Set Solution Problem Understanding The problem is asking us to design a system that tracks tweets by their timestamp and allows querying the number of tweets in specific time intervals, split according to a frequency (minute, hour, or day). Specifically, we need to implement a TweetCounts class with two operations: recording a tweet and retrieving...

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LeetCode 355 - Design Twitter

LeetCode Problem 355 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Heap (Priority Queue) Solution Problem Understanding This problem asks us to design a simplified version of a social media platform similar to Twitter. The system must support four main operations: Users can post tweets. Users can follow other users. Users can unfollow other users. Users can retrieve a personalized news feed containing the 10 most recent tweets from...

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LeetCode 2031 - Count Subarrays With More Ones Than Zeros

LeetCode Problem 2031 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Divide and Conquer, Binary Indexed Tree, Segment Tree, Merge Sort, Ordered Set Solution Problem Understanding This problem asks us to count all subarrays of a binary array nums that contain more 1 s than 0 s. In other words, for any contiguous slice of the array, if the number of 1 s exceeds the number of 0 s,...

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LeetCode 349 - Intersection of Two Arrays

LeetCode Problem 349 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Binary Search, Sorting Solution Problem Understanding The problem gives us two integer arrays, nums1 and nums2 , and asks us to return their intersection. The intersection consists of all values that appear in both arrays. However, the result must contain only unique elements, even if a number appears many times in either array. For example, if nums1 =...

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LeetCode 73 - Set Matrix Zeroes

LeetCode Problem 73 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution LeetCode 73, Set Matrix Zeroes Problem Understanding The problem gives us a two dimensional matrix of integers with m rows and n columns. We must modify the matrix in place so that whenever a cell contains 0 , every element in that cell's entire row and entire column also becomes 0 . The important detail is that the...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 149 - Max Points on a Line

LeetCode Problem 149 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Geometry Solution Problem Understanding The problem gives us a list of points on a 2D coordinate plane. Each point is represented as [x, y] , where x and y are integer coordinates. Our task is to determine the maximum number of points that lie on the same straight line. In other words, among all possible lines that can be...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 219 - Contains Duplicate II

LeetCode Problem 219 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sliding Window Solution Problem Understanding The problem asks us to determine whether an array contains two equal values whose indices are close to each other. More specifically, we need to check if there exist two distinct indices i and j such that: nums[i] == nums[j] abs(i - j) <= k The array nums contains integers, and k defines the maximum...

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LeetCode 645 - Set Mismatch

LeetCode Problem 645 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation, Sorting Solution Problem Understanding The problem describes a corrupted set of integers. Originally, the set contained every number from 1 through n exactly once. Because of an error, one number was duplicated, which means one other number disappeared. The input array nums represents this corrupted version of the set. Since one number appears twice and another is missing,...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1743 - Restore the Array From Adjacent Pairs

LeetCode Problem 1743 Difficulty: 🟡 Medium Topics: Array, Hash Table, Depth-First Search Solution Problem Understanding The problem gives us all adjacent pairs from an unknown array nums , and our task is to reconstruct the original array. Suppose the original array was: [1, 2, 3, 4] The adjacent pairs would be: [1,2], [2,3], [3,4] The important detail is that the pairs can appear in any order and can also be...

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LeetCode 1546 - Maximum Number of Non-Overlapping Subarrays With Sum Equals Target

LeetCode Problem 1546 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Prefix Sum Solution Problem Understanding This problem asks us to find the maximum number of non-overlapping subarrays in a given integer array nums such that the sum of each subarray equals a given target . In simpler terms, we are scanning the array to find consecutive sequences of numbers whose sum matches target and ensuring that no two chosen...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 987 - Vertical Order Traversal of a Binary Tree

LeetCode Problem 987 Difficulty: 🔴 Hard Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Sorting, Binary Tree Solution Problem Understanding The problem asks us to compute the vertical order traversal of a binary tree. In simpler terms, we are asked to “look at the tree from the side” and collect the nodes that align in the same vertical column. Each node has a position defined by (row, col) where row...

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LeetCode 1181 - Before and After Puzzle

LeetCode Problem 1181 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us an array of phrases, where each phrase is a string made of lowercase English letters and spaces. Every phrase is well-formed, meaning there are no leading or trailing spaces and no consecutive spaces. We must generate all possible "Before and After" puzzles by combining two different phrases. A valid merge happens...

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LeetCode 895 - Maximum Frequency Stack

LeetCode Problem 895 Difficulty: 🔴 Hard Topics: Hash Table, Stack, Design, Ordered Set Solution Problem Understanding The problem is asking us to design a custom stack-like data structure that supports two operations: pushing values onto the stack and popping the most frequent element. A key difference from a traditional stack is that the pop operation does not necessarily remove the element at the top; instead, it removes the element with...

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LeetCode 1124 - Longest Well-Performing Interval

LeetCode Problem 1124 Difficulty: 🟡 Medium Topics: Array, Hash Table, Stack, Monotonic Stack, Prefix Sum Solution Edit LeetCode 1124 - Longest Well-Performing Interval Problem Understanding The problem gives us an array hours , where each element represents the number of hours an employee worked on a specific day. A day is classified as tiring if the employee worked strictly more than 8 hours . Otherwise, the day is considered non-tiring...

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LeetCode 1540 - Can Convert String in K Moves

LeetCode Problem 1540 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem is asking whether it is possible to convert one string s into another string t using at most k moves, following specific shifting rules. Each move allows you to choose a character in s that has not been moved before and shift it forward in the alphabet exactly i times, where i is the move...

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LeetCode 1636 - Sort Array by Increasing Frequency

LeetCode Problem 1636 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to reorder an array of integers based on the frequency of each value. Specifically, elements with lower frequency should appear before elements with higher frequency. If two elements have the same frequency, the one with the larger value should appear first. In other words, the sort is primary by increasing frequency and...

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LeetCode 311 - Sparse Matrix Multiplication

LeetCode Problem 311 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution Problem Understanding This problem asks us to multiply two matrices, but with an important detail: both matrices are sparse. A sparse matrix is a matrix where most entries are zero. The goal is to take advantage of this property so that we avoid unnecessary computation involving zero values. We are given: mat1 , an m x k matrix...

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LeetCode 952 - Largest Component Size by Common Factor

LeetCode Problem 952 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Union-Find, Number Theory Solution Problem Understanding The problem asks us to find the largest connected component in a graph derived from an array of unique positive integers nums . Each integer in the array represents a node, and there is an undirected edge between two nodes if the corresponding numbers share a common factor greater than 1. Essentially, numbers...

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LeetCode 1297 - Maximum Number of Occurrences of a Substring

LeetCode Problem 1297 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Please provide the missing problem number or title you want the guide for. Your prompt contains the formatting/template and problem statement for LeetCode 1297 - Maximum Number of Occurrences of a Substring , so I assume you want the complete technical solution guide for that problem. If yes, I will provide the full guide with all required...

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LeetCode 519 - Random Flip Matrix

LeetCode Problem 519 Difficulty: 🟡 Medium Topics: Hash Table, Math, Reservoir Sampling, Randomized Solution Problem Understanding The problem gives us an m x n matrix where every cell initially contains 0 . We need to design a data structure that supports two operations efficiently: flip() This operation must randomly choose one of the remaining cells that still contains 0 , return its coordinates [row, col] , and then change that...

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LeetCode 1660 - Correct a Binary Tree

LeetCode Problem 1660 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Here is a complete, detailed technical solution guide for LeetCode 1660 - Correct a Binary Tree , formatted exactly as requested. Problem Understanding The problem presents a binary tree that has a subtle defect: there is exactly one invalid node , whose right child incorrectly points to another node on the same depth...

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LeetCode 828 - Count Unique Characters of All Substrings of a Given String

LeetCode Problem 828 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming Solution Problem Understanding The problem asks us to compute the total number of unique characters across every possible substring of a given string. A character is considered unique inside a substring if it appears exactly once within that substring. For example, in the substring "ABA" , only "B" is unique because "A" appears twice. We are given a...

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LeetCode 863 - All Nodes Distance K in Binary Tree

LeetCode Problem 863 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution LeetCode 863 - All Nodes Distance K in Binary Tree Problem Understanding This problem gives us the root of a binary tree, a specific target node inside that tree, and an integer k . Our task is to return all node values whose distance from the target node is exactly k . The...

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LeetCode 720 - Longest Word in Dictionary

LeetCode Problem 720 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, Sorting Solution LeetCode 720, Longest Word in Dictionary Problem Understanding This problem gives us an array of lowercase English words and asks us to find the longest word that can be constructed one character at a time using other words from the same dictionary. More specifically, suppose we have a candidate word like "world" . For this word...

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LeetCode 833 - Find And Replace in String

LeetCode Problem 833 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us a string s and three parallel arrays: indices[i] tells us where a replacement might happen sources[i] is the substring we expect to find at that index targets[i] is the string we should replace it with if the match is valid For each operation, we must check whether sources[i] actually appears in...

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LeetCode 1138 - Alphabet Board Path

LeetCode Problem 1138 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem gives us a special alphabet board and asks us to generate the shortest sequence of moves needed to spell a target string. The board contains lowercase English letters arranged in rows: abcde fghij klmno pqrst uvwxy z We always begin at position (0, 0) , which corresponds to the character 'a' . From any valid...

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LeetCode 997 - Find the Town Judge

LeetCode Problem 997 Difficulty: 🟢 Easy Topics: Array, Hash Table, Graph Theory Solution Problem Understanding The problem describes a town containing n people labeled from 1 to n . Among these people, there may exist a special person called the town judge. The judge must satisfy two strict conditions: The judge trusts nobody. Every other person trusts the judge. Additionally, the problem guarantees that if such a person exists, there...

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LeetCode 268 - Missing Number

LeetCode Problem 268 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Binary Search, Bit Manipulation, Sorting Solution Problem Understanding The problem gives an array nums containing n distinct integers. Every number is supposed to come from the range [0, n] , which means there are actually n + 1 possible values in total. Since the array only contains n numbers, exactly one value from that range is missing. The task...

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LeetCode 726 - Number of Atoms

LeetCode Problem 726 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Sorting Solution Problem Understanding This problem asks us to parse a chemical formula represented as a string and return a canonical string that counts the number of each atom in the formula. Each atom starts with an uppercase letter and may have lowercase letters following it. A number following an atom represents how many of that atom are present,...

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LeetCode 2034 - Stock Price Fluctuation

LeetCode Problem 2034 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Data Stream, Ordered Set Solution Here’s a complete, detailed technical solution guide for LeetCode 2034 - Stock Price Fluctuation following your formatting and content requirements. Problem Understanding The problem presents a stream of stock price records where each record consists of a timestamp and a price . However, the stream is not guaranteed to be chronological, and...

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LeetCode 533 - Lonely Pixel II

LeetCode Problem 533 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution Problem Understanding The problem gives us a matrix called picture , where each cell contains either 'B' for a black pixel or 'W' for a white pixel. We are also given an integer target . We need to count how many black pixels qualify as "lonely pixels" under two strict conditions. Suppose a black pixel is located at...

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LeetCode 928 - Minimize Malware Spread II

LeetCode Problem 928 Difficulty: 🔴 Hard Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Graph Theory Solution Problem Understanding The problem is about a network of computers represented as an undirected graph using an adjacency matrix. Each node represents a computer, and an edge between two nodes indicates a direct connection. Some nodes are initially infected with malware, and the infection spreads along the network: if a node is...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 381 - Insert Delete GetRandom O(1) - Duplicates allowed

LeetCode Problem 381 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Design, Randomized Solution Problem Understanding The problem asks us to design a data structure called RandomizedCollection that behaves like a multiset. Unlike a normal set, duplicate values are allowed. The data structure must support three operations: Insert a value Remove one occurrence of a value Return a random element The critical requirement is that every operation must run in...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1788 - Maximize the Beauty of the Garden

LeetCode Problem 1788 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Prefix Sum Solution Problem Understanding We are given an integer array flowers , where each element represents the beauty value of a flower in a line. We may remove any flowers we want, while preserving the relative order of the remaining flowers. After removals, the remaining sequence must form a valid garden. A valid garden must satisfy two conditions:...

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LeetCode 1218 - Longest Arithmetic Subsequence of Given Difference

LeetCode Problem 1218 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming Solution Problem Understanding The problem asks for the length of the longest arithmetic subsequence in a given array arr such that the difference between consecutive elements equals a specified integer difference . A subsequence can skip elements but must preserve the original order. In other words, for each element arr[i] in the array, we want to know how...

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LeetCode 379 - Design Phone Directory

LeetCode Problem 379 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List, Design, Queue Solution Problem Understanding The problem asks us to design a data structure that manages a fixed pool of phone numbers. Initially, every number from 0 to maxNumbers - 1 is available. The directory must support three operations efficiently. The get() operation should assign and return an available number. If no numbers remain, it should return -1...

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LeetCode 804 - Unique Morse Code Words

LeetCode Problem 804 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to determine how many unique Morse code transformations exist among a list of words. Each lowercase English letter maps to a specific Morse code representation. A word can be transformed into Morse code by replacing each character with its Morse equivalent and concatenating the results together into a single string. For example,...

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LeetCode 1128 - Number of Equivalent Domino Pairs

LeetCode Problem 1128 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem is asking us to count all pairs of dominoes in a list that are equivalent, where two dominoes [a, b] and [c, d] are considered equivalent if one is a rotation of the other, meaning either (a == c and b == d) or (a == d and b == c) . In other...

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LeetCode 1170 - Compare Strings by Frequency of the Smallest Character

LeetCode Problem 1170 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Binary Search, Sorting Solution Problem Understanding The problem defines a function f(s) for a non-empty string s . This function returns the frequency of the lexicographically smallest character in the string. For example: f("dcce") = 2 The smallest character is 'c' 'c' appears 2 times We are given two arrays: queries , containing query strings words , containing reference...

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LeetCode 142 - Linked List Cycle II

LeetCode Problem 142 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Two Pointers Solution Problem Understanding This problem asks us to determine whether a singly linked list contains a cycle and, if it does, return the exact node where that cycle begins. A linked list is normally a sequence of nodes where each node points to the next one, eventually ending with null . However, in this problem, the next...

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LeetCode 1001 - Grid Illumination

LeetCode Problem 1001 Difficulty: 🔴 Hard Topics: Array, Hash Table Solution Problem Understanding This problem models a very large chessboard-like grid where certain cells contain active lamps. A lamp illuminates four directions simultaneously: Its entire row Its entire column Its main diagonal, identified by row - col Its anti-diagonal, identified by row + col For every query, we must determine whether the queried cell is illuminated by at least one...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1742 - Maximum Number of Balls in a Box

LeetCode Problem 1742 Difficulty: 🟢 Easy Topics: Hash Table, Math, Counting Solution Problem Understanding The problem describes a scenario in which you have a sequence of balls numbered consecutively from lowLimit to highLimit , inclusive. Each ball must be placed into a box, where the box number is determined by the sum of the digits of the ball’s number. For example, ball 321 goes into box 3 + 2 +...

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LeetCode 1160 - Find Words That Can Be Formed by Characters

LeetCode Problem 1160 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution Problem Understanding This problem asks us to determine which words from a given list can be constructed using the characters available in another string, chars . A word is considered good if every character it needs exists in chars in sufficient quantity. Importantly, characters in chars can only be used once per word , meaning repeated letters...

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LeetCode 380 - Insert Delete GetRandom O(1)

LeetCode Problem 380 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Design, Randomized Solution Problem Understanding This problem asks us to design a custom data structure called RandomizedSet that supports three operations: Insert a value into the set. Remove a value from the set. Return a random element from the set. The critical requirement is that all three operations must run in average O(1) time complexity. The behavior is very...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 267 - Palindrome Permutation II

LeetCode Problem 267 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution Problem Understanding The problem asks us to generate every unique palindrome that can be formed by rearranging the characters of a given string. A palindrome is a string that reads the same forward and backward. For example, "abba" and "racecar" are palindromes. The task is not to determine whether the original string is already a palindrome. Instead, we...

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LeetCode 1213 - Intersection of Three Sorted Arrays

LeetCode Problem 1213 Difficulty: 🟢 Easy Topics: Array, Hash Table, Binary Search, Counting Solution Problem Understanding The problem gives us three integer arrays, arr1 , arr2 , and arr3 . Each array is already sorted in strictly increasing order. Strictly increasing means there are no duplicate values inside the same array, and every next element is larger than the previous one. Our task is to return a new sorted array...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 966 - Vowel Spellchecker

LeetCode Problem 966 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to build a spellchecker with three levels of matching priority. For every query word, we must search the wordlist and return the best matching word according to a strict precedence order. The first and highest priority rule is an exact match. If the query exactly matches a word in the wordlist ,...

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LeetCode 159 - Longest Substring with At Most Two Distinct Characters

LeetCode Problem 159 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest contiguous substring in a string s that contains at most two distinct characters. A substring is a continuous sequence of characters inside the original string. The phrase "at most two distinct characters" means the substring may contain one unique character or two unique characters,...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 127 - Word Ladder

LeetCode Problem 127 Difficulty: 🔴 Hard Topics: Hash Table, String, Breadth-First Search Solution Problem Understanding This problem asks us to determine the length of the shortest transformation sequence between two words, beginWord and endWord , under a strict transformation rule. A valid transformation changes exactly one character at a time , and every intermediate word must exist inside the provided wordList . The beginWord itself does not need to appear...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 496 - Next Greater Element I

LeetCode Problem 496 Difficulty: 🟢 Easy Topics: Array, Hash Table, Stack, Monotonic Stack Solution Problem Understanding The problem is asking us to find the next greater element for each element in nums1 within another array nums2 . Formally, for each element in nums1 , we need to locate its position in nums2 and then find the first element to its right in nums2 that is greater than itself . If...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1096 - Brace Expansion II

LeetCode Problem 1096 Difficulty: 🔴 Hard Topics: Hash Table, String, Backtracking, Stack, Breadth-First Search, Sorting Solution Problem Understanding The problem asks us to interpret a string expression representing a set of words generated according to a specific grammar and return all distinct words sorted in lexicographical order. The expression can contain lowercase letters, curly braces {} , and commas , . The rules of the grammar are: A single lowercase...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1152 - Analyze User Website Visit Pattern

LeetCode Problem 1152 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Here is a comprehensive, detailed technical solution guide for LeetCode 1152 - Analyze User Website Visit Pattern following your requested format exactly. Problem Understanding The problem asks us to analyze user website visit sequences to determine the most common pattern of length three. We are given three arrays of equal length: username , timestamp , and website...

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LeetCode 347 - Top K Frequent Elements

LeetCode Problem 347 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect Solution Problem Understanding The problem asks us to return the k most frequently occurring elements from an integer array. We are given an array nums , which may contain duplicates, and an integer k . Our task is to identify which values appear most often and return exactly k...

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LeetCode 953 - Verifying an Alien Dictionary

LeetCode Problem 953 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem asks us to verify whether a list of words is sorted according to a custom alphabet order, instead of the normal English alphabetical order. In normal lexicographical ordering, characters are compared from left to right using the standard alphabet. Here, the alien language still uses lowercase English letters, but the relative order of those...

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LeetCode 1090 - Largest Values From Labels

LeetCode Problem 1090 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Counting Solution Problem Understanding The problem gives us two arrays, values and labels , where each index represents a single item. The item at index i has a value values[i] and a category or group identifier labels[i] . We want to choose a subset of these items such that two constraints are satisfied: We can select at most...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 106 - Construct Binary Tree from Inorder and Postorder Traversal

LeetCode Problem 106 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Tree, Binary Tree Solution Problem Understanding The problem provides two traversal orders of the same binary tree: inorder , which follows the order: left subtree, root, right subtree postorder , which follows the order: left subtree, right subtree, root We must reconstruct the original binary tree and return its root node. The key challenge is understanding how...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1122 - Relative Sort Array

LeetCode Problem 1122 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Counting Sort Solution Problem Understanding This problem asks us to sort arr1 , but not in ordinary ascending or descending order. Instead, the sorting order is partially dictated by another array, arr2 . The key requirement is that every number appearing in arr2 must appear in arr1 in exactly the same relative order. If a number appears multiple times...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1207 - Unique Number of Occurrences

LeetCode Problem 1207 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 1207 - Unique Number of Occurrences Problem Understanding The problem gives us an integer array arr and asks whether every distinct number appears a unique number of times. In other words, we first count how many times each value occurs in the array. After we have all the frequencies, we must verify that no two different values share...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1814 - Count Nice Pairs in an Array

LeetCode Problem 1814 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem gives us an array of non-negative integers called nums . We need to count how many index pairs (i, j) satisfy the following conditions: i < j nums[i] + rev(nums[j]) == nums[j] + rev(nums[i]) Here, rev(x) means reversing the digits of the integer. For example: rev(123) = 321 rev(120) = 21 The goal...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1177 - Can Make Palindrome from Substring

LeetCode Problem 1177 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Bit Manipulation, Prefix Sum Solution Problem Understanding The problem asks us to determine whether certain substrings of a given string s can be rearranged and partially modified to form a palindrome. A palindrome is a string that reads the same forward and backward. For each query [lefti, righti, ki] , we are allowed to take the substring s[lefti...righti] ,...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 734 - Sentence Similarity

LeetCode Problem 734 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us two sentences, where each sentence is represented as an array of words. We are also given a list of word pairs that define which words are considered similar. Our task is to determine whether the two sentences are similar according to the following rules: The sentences must contain the same number of...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 694 - Number of Distinct Islands

LeetCode Problem 694 Difficulty: 🟡 Medium Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Sorting, Matrix, Hash Function Solution Problem Understanding The problem gives us a binary matrix called grid , where each cell contains either 0 or 1 . A value of 1 represents land, and a value of 0 represents water. An island is formed by connecting adjacent land cells in the four cardinal directions: up, down,...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 169 - Majority Element

LeetCode Problem 169 Difficulty: 🟢 Easy Topics: Array, Hash Table, Divide and Conquer, Sorting, Counting Solution Problem Understanding The problem asks us to identify the majority element in a given array of integers nums . The majority element is defined as the number that appears more than half of the times in the array, i.e., more than ⌊n / 2⌋ times where n is the length of the array. The...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 433 - Minimum Genetic Mutation

LeetCode Problem 433 Difficulty: 🟡 Medium Topics: Hash Table, String, Breadth-First Search Solution Problem Understanding The problem describes a mutation process between genetic sequences. Each gene is represented as a string of exactly 8 characters, and every character must be one of four possible nucleotides: 'A' , 'C' , 'G' , or 'T' . We are given three inputs: startGene , the initial gene sequence endGene , the target gene...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 423 - Reconstruct Original Digits from English

LeetCode Problem 423 Difficulty: 🟡 Medium Topics: Hash Table, Math, String Solution Problem Understanding The problem asks us to reconstruct digits from a jumbled string of English letters representing numbers from 0 to 9 . The input string s contains only characters from the set ["e","g","f","i","h","o","n","s","r","u","t","w","v","x","z"] , which correspond to the letters used in English words for digits ( "zero" , "one" , "two" , etc.). The order of the...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1394 - Find Lucky Integer in an Array

LeetCode Problem 1394 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding This problem asks us to identify a special number in an array called a lucky integer . A lucky integer is defined as an integer in the array whose frequency exactly matches its value. For example, if the number 3 appears exactly 3 times in the array, it is considered lucky. Our task is to find...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 616 - Add Bold Tag in String

LeetCode Problem 616 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, String Matching Solution Problem Understanding The problem asks us to insert HTML-style bold tags into a string whenever a substring matches any word from a given dictionary of words. We are given: A string s An array words , where each element is a pattern we want to highlight For every occurrence of every word inside s ,...

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LeetCode 1010 - Pairs of Songs With Total Durations Divisible by 60

LeetCode Problem 1010 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find pairs of songs whose total duration is divisible by 60. Specifically, we are given an array time where each element represents the length of a song in seconds. We need to count all unique pairs (i, j) where i < j and (time[i] + time[j]) % 60 == 0 ....

Practice › LeetCode › LeetCode 01xx ›

LeetCode 160 - Intersection of Two Linked Lists

LeetCode Problem 160 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Two Pointers Solution LeetCode 160, Intersection of Two Linked Lists Problem Understanding This problem asks us to determine whether two singly linked lists share a common node, and if they do, return the exact node where the intersection begins. The important detail is that an intersection is based on node reference, not node value. Two nodes containing the same...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2023 - Number of Pairs of Strings With Concatenation Equal to Target

LeetCode Problem 2023 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to find the number of ordered pairs of indices (i, j) in a list of digit strings nums such that concatenating nums[i] and nums[j] produces exactly the string target . It is important that i and j are distinct , meaning the same element cannot pair with itself in the same...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 575 - Distribute Candies

LeetCode Problem 575 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding This problem asks us to maximize the number of distinct candy types Alice can eat while respecting a strict limit on how many candies she is allowed to consume. We are given an integer array candyType , where each element represents the type of a candy. Multiple candies may share the same type value. The total number...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1284 - Minimum Number of Flips to Convert Binary Matrix to Zero Matrix

LeetCode Problem 1284 Difficulty: 🔴 Hard Topics: Array, Hash Table, Bit Manipulation, Breadth-First Search, Matrix Solution Problem Understanding The problem asks us to transform a given binary matrix mat into a zero matrix, where all elements are 0 . Each operation allows you to flip a cell and its four immediate neighbors (up, down, left, right). Flipping means changing a 1 to 0 or a 0 to 1 . The...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1358 - Number of Substrings Containing All Three Characters

LeetCode Problem 1358 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count all substrings of a given string s that contain at least one occurrence of each character 'a' , 'b' , and 'c' . The string s is guaranteed to contain only these three characters. Essentially, we need to find the number of contiguous sequences in s where none of...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 846 - Hand of Straights

LeetCode Problem 846 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting Solution Problem Understanding The problem asks us to determine whether a given collection of cards can be rearranged into groups of consecutive numbers, each of size groupSize . Each card is represented by an integer in the array hand , and Alice wants to form groups such that within each group, the card numbers are consecutive and the...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 582 - Kill Process

LeetCode Problem 582 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Breadth-First Search Solution Problem Understanding This problem models processes in an operating system as a tree structure. Every process has exactly one parent, except for the root process, which has no parent and is identified by ppid[i] = 0 . The input is given through two parallel arrays: pid[i] represents the ID of a process ppid[i] represents...

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LeetCode 1371 - Find the Longest Substring Containing Vowels in Even Counts

LeetCode Problem 1371 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum Solution LeetCode 1371 - Find the Longest Substring Containing Vowels in Even Counts Problem Understanding This problem asks us to find the length of the longest contiguous substring in which every vowel appears an even number of times. The vowels we care about are: a e i o u A vowel may appear zero times, and...

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LeetCode 1542 - Find Longest Awesome Substring

LeetCode Problem 1542 Difficulty: 🔴 Hard Topics: Hash Table, String, Bit Manipulation Solution Problem Understanding The problem asks us to find the length of the longest awesome substring in a given string s consisting of digits. A substring is considered awesome if it can be rearranged (via any number of swaps) to form a palindrome. In other words, the characters in the substring can be permuted so that it reads...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 676 - Implement Magic Dictionary

LeetCode Problem 676 Difficulty: 🟡 Medium Topics: Hash Table, String, Depth-First Search, Design, Trie Solution Problem Understanding The problem asks us to design a dictionary-like data structure that supports two operations. First, we load a collection of distinct words into the structure. Second, given a query string, we determine whether it is possible to modify exactly one character in the query so that the resulting string matches one of the...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 325 - Maximum Size Subarray Sum Equals k

LeetCode Problem 325 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to find the longest contiguous subarray whose sum is exactly equal to a given integer k . A subarray is a continuous portion of the array. This means we cannot reorder elements or skip positions. We must choose a start index and an end index such that every element between them...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 187 - Repeated DNA Sequences

LeetCode Problem 187 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Sliding Window, Rolling Hash, Hash Function Solution Problem Understanding This problem asks us to find all repeated DNA subsequences of a fixed length, specifically length 10 , inside a given DNA string. The input is a string s , where each character represents a DNA nucleotide. The string contains only four possible characters: 'A' 'C' 'G' 'T' We...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 756 - Pyramid Transition Matrix

LeetCode Problem 756 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking, Bit Manipulation Solution Here’s a full, detailed technical solution guide for LeetCode 756 - Pyramid Transition Matrix , following your formatting rules precisely. Problem Understanding The problem is asking us to determine whether a pyramid of blocks can be constructed from a given bottom row of blocks, such that all blocks above obey certain triangular stacking rules. Each block...

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LeetCode 1836 - Remove Duplicates From an Unsorted Linked List

LeetCode Problem 1836 Difficulty: 🟡 Medium Topics: Hash Table, Linked List Solution Problem Understanding This problem gives us the head of a singly linked list whose values are not sorted. Our task is to remove every node whose value appears more than once anywhere in the list. The important detail is that we are not removing duplicate occurrences while keeping one copy. Instead, if a value appears multiple times, all...

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LeetCode 523 - Continuous Subarray Sum

LeetCode Problem 523 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Prefix Sum Solution Problem Understanding The problem asks us to determine whether an array contains a continuous subarray whose sum is a multiple of k , while also satisfying an important constraint: the subarray must contain at least two elements . In simpler terms, we need to find whether there exists some contiguous segment of nums , with length...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 869 - Reordered Power of 2

LeetCode Problem 869 Difficulty: 🟡 Medium Topics: Hash Table, Math, Sorting, Counting, Enumeration Solution Problem Understanding The problem gives us a positive integer n and asks whether its digits can be rearranged to form a power of two. The rearrangement may keep the digits in their original order or place them in any other order, but the resulting number cannot contain a leading zero. A power of two is any...

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LeetCode 532 - K-diff Pairs in an Array

LeetCode Problem 532 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Binary Search, Sorting Solution Problem Understanding The problem asks us to count how many unique pairs of integers in the array have an absolute difference equal to k . A pair is considered valid if: The two elements come from different indices. The absolute difference between the two values is exactly k . The important detail is that...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1647 - Minimum Deletions to Make Character Frequencies Unique

LeetCode Problem 1647 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting Solution Problem Understanding The problem asks us to make a string good , which means no two distinct characters in the string have the same frequency. The input is a string s consisting of lowercase English letters, and the output is the minimum number of character deletions required to achieve a good string. The frequency of a character...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 305 - Number of Islands II

LeetCode Problem 305 Difficulty: 🔴 Hard Topics: Array, Hash Table, Union-Find Solution Problem Understanding The problem gives us an initially empty m x n grid where every cell starts as water. We are then given a sequence of operations in positions , where each operation turns a specific cell from water into land. After every operation, we must determine how many islands currently exist in the grid. An island is...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 138 - Copy List with Random Pointer

LeetCode Problem 138 Difficulty: 🟡 Medium Topics: Hash Table, Linked List Solution Problem Understanding This problem asks us to create a completely independent copy of a linked list where each node contains two pointers: next , which points to the next node in the list random , which can point to any node in the list or null The key requirement is that the copied list must be a true...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1436 - Destination City

LeetCode Problem 1436 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding In this problem, we are given a list of directed paths between cities. Each entry in paths has the form [cityA, cityB] , which means there is a one way path from cityA to cityB . The graph formed by these paths is guaranteed to represent a straight travel chain with no cycles. This guarantee is...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1593 - Split a String Into the Max Number of Unique Substrings

LeetCode Problem 1593 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution LeetCode 1593 - Split a String Into the Max Number of Unique Substrings Problem Understanding This problem asks us to split a string into the largest possible number of substrings such that every substring is unique. The substrings must be contiguous pieces of the original string, and when combined in order, they must reconstruct the original string exactly....

Practice › LeetCode › LeetCode 09xx ›

LeetCode 992 - Subarrays with K Different Integers

LeetCode Problem 992 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sliding Window, Counting Solution Problem Understanding The problem asks us to count how many contiguous subarrays contain exactly k distinct integers. We are given: An integer array nums An integer k A subarray is any continuous portion of the array. For each possible subarray, we need to determine whether it contains exactly k unique values. If it does, it contributes...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1590 - Make Sum Divisible by P

LeetCode Problem 1590 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding This problem asks us to determine the smallest contiguous subarray we can remove from a given array of positive integers, nums , so that the sum of the remaining elements becomes divisible by a given integer p . The key here is that the subarray must be contiguous, and it cannot be the entire array....

Practice › LeetCode › LeetCode 07xx ›

LeetCode 758 - Bold Words in String

LeetCode Problem 758 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, String Matching Solution Problem Understanding The problem gives us two inputs: an array of strings called words , and a target string s . Every occurrence of every word from words inside s must become bold by surrounding that substring with <b> and </b> tags. The important requirement is that we must use the minimum number of tags...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 146 - LRU Cache

LeetCode Problem 146 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design an efficient cache that follows the Least Recently Used, or LRU, eviction policy. A cache stores a limited number of key-value pairs. When the cache reaches its maximum capacity and a new item must be inserted, the least recently used item should be removed first. The class...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1748 - Sum of Unique Elements

LeetCode Problem 1748 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem gives us an integer array nums and asks us to compute the sum of all elements that appear exactly once in the array. An element is considered unique only if its frequency is exactly one. If a value appears two or more times, it must not contribute to the final sum at all. For...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 706 - Design HashMap

LeetCode Problem 706 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Design, Hash Function Solution Problem Understanding The problem requires us to implement a HashMap from scratch without using any built-in hash table libraries. In other words, we need to create a data structure that can store key-value pairs, allow efficient insertion, retrieval, and deletion of values based on their keys. The input operations consist of put , get...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 391 - Perfect Rectangle

LeetCode Problem 391 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Geometry, Sweep Line Solution Problem Understanding This problem asks whether a collection of smaller axis-aligned rectangles perfectly forms one larger rectangle, with no gaps and no overlaps. Each rectangle is represented as: [xi, yi, ai, bi] where: (xi, yi) is the bottom-left corner (ai, bi) is the top-right corner Because the rectangles are axis-aligned, their sides are parallel to...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 930 - Binary Subarrays With Sum

LeetCode Problem 930 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window, Prefix Sum Solution Problem Understanding The problem asks us to count the number of contiguous subarrays in a binary array nums whose sum equals a given integer goal . A binary array only contains 0 s and 1 s. A subarray is any contiguous sequence of elements in the array, meaning that order matters and elements cannot be...

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LeetCode 2009 - Minimum Number of Operations to Make Array Continuous

LeetCode Problem 2009 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Sliding Window Solution Problem Understanding The problem asks us to transform a given integer array nums into a continuous array with the minimum number of operations. A continuous array is defined by two conditions: all elements are unique, and the difference between the maximum and minimum element equals the length of the array minus one. Essentially, a continuous...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 395 - Longest Substring with At Least K Repeating Characters

LeetCode Problem 395 Difficulty: 🟡 Medium Topics: Hash Table, String, Divide and Conquer, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest substring in which every character appears at least k times. A substring is a contiguous sequence of characters inside the original string. For any candidate substring, we must verify that every distinct character inside that substring has frequency greater than or...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 105 - Construct Binary Tree from Preorder and Inorder Traversal

LeetCode Problem 105 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Tree, Binary Tree Solution Problem Understanding The problem is asking us to reconstruct a binary tree given two traversal orders: preorder and inorder . In a preorder traversal, nodes are visited in the order: root, left subtree, right subtree. In an inorder traversal, nodes are visited in the order: left subtree, root, right subtree. Given these two...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1331 - Rank Transform of an Array

LeetCode Problem 1331 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to replace every number in the input array with its rank when the array is sorted in ascending order. A rank is defined by the relative ordering of the values: The smallest unique value gets rank 1 The second smallest unique value gets rank 2 Equal values must receive the same rank...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1275 - Find Winner on a Tic Tac Toe Game

LeetCode Problem 1275 Difficulty: 🟢 Easy Topics: Array, Hash Table, Matrix, Simulation Solution Problem Understanding This problem asks us to simulate a game of Tic-Tac-Toe on a 3 x 3 grid given a sequence of moves. Each move specifies the row and column where the current player places their mark. Player A always plays first and uses 'X' , while player B uses 'O' . The goal is to determine...

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LeetCode 1296 - Divide Array in Sets of K Consecutive Numbers

LeetCode Problem 1296 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting Solution Problem Understanding The problem gives us an integer array nums and an integer k . We need to determine whether it is possible to divide all numbers in the array into groups where: Each group contains exactly k numbers The numbers inside each group are consecutive integers Every element in the array must belong to exactly one...

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LeetCode 840 - Magic Squares In Grid

LeetCode Problem 840 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Matrix Solution Problem Understanding The problem asks us to count the number of 3 x 3 magic squares inside a larger grid. A magic square is defined as a square where all numbers are distinct integers from 1 to 9 and the sum of each row, column, and the two diagonals is the same. The input is a 2D...

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LeetCode 1442 - Count Triplets That Can Form Two Arrays of Equal XOR

LeetCode Problem 1442 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Bit Manipulation, Prefix Sum Solution Problem Understanding The problem asks us to find the number of triplets (i, j, k) in an array arr such that the XOR of elements from index i to j-1 equals the XOR of elements from index j to k . Formally, for 0 <= i < j <= k < arr.length , define:...

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LeetCode 535 - Encode and Decode TinyURL

LeetCode Problem 535 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Hash Function Solution Problem Understanding The problem asks us to implement a URL shortening service, similar to TinyURL. Given a long URL, our system should generate a short, unique URL that maps back to the original URL. When the short URL is accessed, the system should return the original long URL. Essentially, we need a reversible mapping between long...

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LeetCode 1171 - Remove Zero Sum Consecutive Nodes from Linked List

LeetCode Problem 1171 Difficulty: 🟡 Medium Topics: Hash Table, Linked List Solution Problem Understanding The problem requires us to repeatedly remove consecutive nodes in a singly-linked list whose values sum to zero. The input is the head of a singly-linked list, where each node has an integer value in the range -1000 to 1000 , and the list length ranges from 1 to 1000 nodes. The output should be the...

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LeetCode 1733 - Minimum Number of People to Teach

LeetCode Problem 1733 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy Solution Problem Understanding This problem models communication in a social network where each user knows one or more languages. Two users can communicate directly if they share at least one common language. The goal is to ensure that every friendship pair can communicate after teaching exactly one chosen language to some users. The input consists of three parts: n...

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LeetCode 1604 - Alert Using Same Key-Card Three or More Times in a One Hour Period

LeetCode Problem 1604 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us two parallel arrays, keyName and keyTime . Each index represents a single key-card usage event. For example: keyName[i] = employee name keyTime[i] = time of usage Together, they describe when a specific employee used their key-card during a single day. Our task is to identify every employee who used their key-card...

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LeetCode 531 - Lonely Pixel I

LeetCode Problem 531 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution Problem Understanding The problem gives us a 2D grid called picture , where each cell contains either: 'B' , representing a black pixel 'W' , representing a white pixel We need to count how many lonely black pixels exist in the matrix. A black pixel is considered lonely if: It is black ( 'B' ) There are no...

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LeetCode 1166 - Design File System

LeetCode Problem 1166 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to design a simplified file system that supports two operations: Creating a new path with an associated integer value. Retrieving the value stored at a path. A path behaves similarly to a directory structure in a real operating system. Every path is composed of components separated by / . For example:...

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LeetCode 1244 - Design A Leaderboard

LeetCode Problem 1244 Difficulty: 🟡 Medium Topics: Hash Table, Design, Sorting Solution LeetCode 1244 - Design A Leaderboard Problem Understanding This problem asks us to design a leaderboard system that supports three operations efficiently. Each player has a unique playerId and an associated score. The leaderboard begins empty, and players can be added dynamically as operations are performed. The addScore(playerId, score) operation either creates a new player with the given...

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LeetCode 1386 - Cinema Seat Allocation

LeetCode Problem 1386 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Bit Manipulation Solution Here is a complete, detailed technical solution guide for LeetCode 1386 - Cinema Seat Allocation in a single, comprehensive response following your formatting rules. Problem Understanding The problem describes a cinema with n rows, each containing exactly ten seats, labelled 1 through 10. Some of these seats are already reserved, given in the input array reservedSeats...

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LeetCode 609 - Find Duplicate File in System

LeetCode Problem 609 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a list of directory descriptions, where each string represents a directory and the files inside it. Each file entry includes both the filename and its content. For example: "root/a 1.txt(abcd) 2.txt(efgh)" means: The directory path is root/a Inside this directory: 1.txt contains "abcd" 2.txt contains "efgh" Our task is to identify files...

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LeetCode 691 - Stickers to Spell Word

LeetCode Problem 691 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Dynamic Programming, Backtracking, Bit Manipulation, Memoization, Bitmask Solution Let's dive deep into a detailed technical solution guide for LeetCode 691, following your formatting rules exactly. Problem Understanding The problem asks us to construct a target string using letters cut from an unlimited supply of given stickers, where each sticker is a lowercase word. You can reuse stickers multiple times...

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LeetCode 525 - Contiguous Array

LeetCode Problem 525 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding This problem asks us to find the maximum length of a contiguous subarray in a binary array where the number of 0 s and 1 s are equal. A contiguous subarray means the elements must appear consecutively in the original array. We are not allowed to rearrange elements or skip indices. The goal is to...

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LeetCode 1624 - Largest Substring Between Two Equal Characters

LeetCode Problem 1624 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to find the length of the longest substring that lies between two identical characters in a given string s . The substring does not include the two identical characters themselves. If no character occurs at least twice, we must return -1 . In other words, we are looking for pairs of identical characters...

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LeetCode 1630 - Arithmetic Subarrays

LeetCode Problem 1630 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to determine whether subarrays of a given array can be rearranged to form an arithmetic sequence. An arithmetic sequence is one where the difference between consecutive elements is constant. The input consists of an array nums of integers and two arrays l and r representing the start and end indices of multiple...

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LeetCode 961 - N-Repeated Element in Size 2N Array

LeetCode Problem 961 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem gives us an integer array nums whose length is exactly 2 * n . Among all the numbers in the array, there are n + 1 distinct values. One special value appears exactly n times, while every other value appears only once. Our task is to identify and return the value that appears repeatedly. Another...

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LeetCode 356 - Line Reflection

LeetCode Problem 356 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math Solution Problem Understanding The problem gives us a collection of points on a 2D plane, where each point is represented as [x, y] . We need to determine whether there exists a vertical line, meaning a line parallel to the y-axis, such that reflecting every point across that line produces the exact same set of points. A vertical reflection...

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LeetCode 290 - Word Pattern

LeetCode Problem 290 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem gives two inputs: a pattern string and a space-separated string of words. We need to determine whether the sequence of words follows the same structure as the sequence of characters in the pattern. A valid match requires a bijection between pattern characters and words. A bijection means the mapping works in both directions: Each character...

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LeetCode 1570 - Dot Product of Two Sparse Vectors

LeetCode Problem 1570 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Design Solution Problem Understanding This problem asks us to efficiently compute the dot product between two sparse vectors. A normal vector is simply an array of numbers. The dot product of two vectors is computed by multiplying corresponding elements and summing the results. For two vectors a and b of the same length: $$a \cdot b = \sum_{i=0}^{n-1}...

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LeetCode 666 - Path Sum IV

LeetCode Problem 666 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Binary Tree Solution Problem Understanding This problem encodes a binary tree into a compact array of three-digit integers. Each integer stores three pieces of information: The hundreds digit represents the depth of the node. The tens digit represents the node's position within that depth level. The units digit represents the node's actual value. For example, the integer...

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LeetCode 1506 - Find Root of N-Ary Tree

LeetCode Problem 1506 Difficulty: 🟡 Medium Topics: Hash Table, Bit Manipulation, Tree, Depth-First Search Solution Problem Understanding This problem gives us every node of an N-ary tree in an arbitrary order, and asks us to determine which node is the root of the tree. An N-ary tree is a tree where each node can have zero or more children. Each node contains a unique integer value and a list of...

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LeetCode 30 - Substring with Concatenation of All Words

LeetCode Problem 30 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find all starting indices in a string s where a substring exists that is a concatenation of all the words in the array words exactly once, without any intervening characters. The words can appear in any order, and all words in words have the same length. The input s is...

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LeetCode 141 - Linked List Cycle

LeetCode Problem 141 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Two Pointers Solution LeetCode 141, Linked List Cycle Problem Understanding This problem asks us to determine whether a singly linked list contains a cycle. A cycle exists when following the next pointers eventually leads back to a node that has already been visited, instead of reaching None . The input is the head node of a singly linked list....

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LeetCode 1198 - Find Smallest Common Element in All Rows

LeetCode Problem 1198 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Matrix, Counting Solution Problem Understanding The problem asks us to find the smallest element that appears in every row of a given m x n matrix mat , where each row is sorted in strictly increasing order. In other words, we need an element that is common to all rows and is the minimum among all such elements....

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LeetCode 1399 - Count Largest Group

LeetCode Problem 1399 Difficulty: 🟢 Easy Topics: Hash Table, Math, Counting Solution Problem Understanding The problem asks us to group all integers from 1 to n based on the sum of their digits. Every number belongs to exactly one group, determined by adding together all of its decimal digits. For example: 14 has digit sum 1 + 4 = 5 5 also has digit sum 5 So both numbers belong...

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LeetCode 1015 - Smallest Integer Divisible by K

LeetCode Problem 1015 Difficulty: 🟡 Medium Topics: Hash Table, Math Solution Problem Understanding The problem asks us to find the length of the smallest positive integer that satisfies two conditions: The integer contains only the digit 1 The integer is divisible by k Such numbers are commonly called repunits. Examples include: 1 11 111 1111 The key detail is that we do not need to return the number itself. We...

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LeetCode 554 - Brick Wall

LeetCode Problem 554 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem gives us a wall made of multiple rows of bricks. Each row is represented as an array of integers, where each integer describes the width of a brick. Every brick has height 1 , and all rows together form a rectangle with the same total width. We want to draw a vertical line from the...

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LeetCode 1525 - Number of Good Ways to Split a String

LeetCode Problem 1525 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Bit Manipulation Solution Problem Understanding The problem asks us to count how many ways we can split a string into two non-empty parts such that both parts contain the same number of distinct characters. A split occurs between two adjacent characters. For a string of length n , there are exactly n - 1 possible split positions because...

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LeetCode 1554 - Strings Differ by One Character

LeetCode Problem 1554 Difficulty: 🟡 Medium Topics: Hash Table, String, Rolling Hash, Hash Function Solution Problem Understanding This problem gives us an array of unique strings called dict , where every string has the same length. We must determine whether there exists at least one pair of strings that differ by exactly one character at the same position. In other words, we are looking for two strings where: Their lengths...

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LeetCode 1460 - Make Two Arrays Equal by Reversing Subarrays

LeetCode Problem 1460 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding This problem asks whether one integer array, arr , can be transformed into another array, target , by repeatedly reversing any non-empty contiguous subarray. A subarray reversal means choosing a continuous segment of the array and reversing the order of its elements. For example, reversing [2,4,1] changes it into [1,4,2] . The key detail is that...

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LeetCode 904 - Fruit Into Baskets

LeetCode Problem 904 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution Problem Understanding This problem asks us to find the length of the longest contiguous subarray that contains at most two distinct values. Each element in the fruits array represents the type of fruit produced by a tree. We are allowed to start at any position and move only to the right, collecting exactly one fruit from every...

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LeetCode 1461 - Check If a String Contains All Binary Codes of Size K

LeetCode Problem 1461 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Rolling Hash, Hash Function Solution Problem Understanding The problem gives us a binary string s and an integer k . A binary string contains only the characters '0' and '1' . We must determine whether every possible binary code of length k appears somewhere inside s as a substring. A binary code of length k is simply any...

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LeetCode 359 - Logger Rate Limiter

LeetCode Problem 359 Difficulty: 🟢 Easy Topics: Hash Table, Design, Data Stream Solution Problem Understanding The problem asks us to design a logging system that controls how frequently identical messages can be printed. Each message is associated with a timestamp, and the same message is only allowed to be printed once every 10 seconds. More specifically, when a message is printed at timestamp t , the next time the exact...

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LeetCode 791 - Custom Sort String

LeetCode Problem 791 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting Solution LeetCode 791, Custom Sort String Problem Understanding The problem gives us two strings, order and s . The string order defines a custom character ordering. Unlike normal alphabetical ordering, the characters in order specify exactly how characters should be prioritized relative to one another. Every character in order is unique, which means there are no conflicts or duplicate...

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LeetCode 1658 - Minimum Operations to Reduce X to Zero

LeetCode Problem 1658 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Sliding Window, Prefix Sum Solution Problem Understanding The problem gives us an integer array nums and a target integer x . In one operation, we are allowed to remove either the leftmost element or the rightmost element from the array. Whenever we remove a value, we subtract it from x . Our goal is to reduce x to...

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LeetCode 594 - Longest Harmonious Subsequence

LeetCode Problem 594 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sliding Window, Sorting, Counting Solution Problem Understanding The problem asks us to find the length of the longest harmonious subsequence in an integer array. A harmonious array is defined as one where the difference between the maximum and minimum values is exactly 1 . In simpler terms, given an array nums , we are searching for the longest collection of...

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LeetCode 1426 - Counting Elements

LeetCode Problem 1426 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to count the number of elements in an array arr for which the value plus one also exists in the array. In other words, for every element x in the array, we check whether x + 1 is also present. If it is, we include x in our count. If duplicates exist, each...

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LeetCode 621 - Task Scheduler

LeetCode Problem 621 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The Task Scheduler problem asks us to determine the minimum number of CPU intervals required to complete a list of tasks with a cooling constraint. Each task is represented by an uppercase letter A-Z. The CPU can execute one task per interval or remain idle. After executing a task, the CPU...

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LeetCode 631 - Design Excel Sum Formula

LeetCode Problem 631 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Graph Theory, Design, Topological Sort, Matrix Solution Problem Understanding The problem asks us to design a miniature spreadsheet system similar to Microsoft Excel. The spreadsheet contains cells arranged in rows and columns, where rows are numbered starting from 1 and columns are labeled using uppercase letters such as A , B , C , and so on. Each cell...

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LeetCode 1781 - Sum of Beauty of All Substrings

LeetCode Problem 1781 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to calculate the sum of beauty for all substrings of a given string s . The beauty of a substring is defined as the difference between the highest frequency and the lowest frequency of any character that occurs in that substring. In simpler terms, for each substring, we count how many times...

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LeetCode 970 - Powerful Integers

LeetCode Problem 970 Difficulty: 🟡 Medium Topics: Hash Table, Math, Enumeration Solution LeetCode 970 - Powerful Integers Problem Understanding This problem asks us to generate all integers that can be written in the form: $x^i + y^j$ where: i >= 0 j >= 0 the resulting value is less than or equal to bound The inputs are three integers: x , the base of the first exponential term y ,...

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LeetCode 133 - Clone Graph

LeetCode Problem 133 Difficulty: 🟡 Medium Topics: Hash Table, Depth-First Search, Breadth-First Search, Graph Theory Solution Clone Graph Problem Understanding This problem asks us to create a deep copy of an undirected connected graph. We are given a reference to one node in the graph, and we must return a completely independent copy of the entire graph structure. Each graph node contains two pieces of information: val , an integer...

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LeetCode 438 - Find All Anagrams in a String

LeetCode Problem 438 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem gives two strings, s and p , both containing only lowercase English letters. We need to find every starting index in s where a substring is an anagram of p . An anagram means the substring contains exactly the same characters as p , with the same frequencies, but possibly in a different...

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LeetCode 266 - Palindrome Permutation

LeetCode Problem 266 Difficulty: 🟢 Easy Topics: Hash Table, String, Bit Manipulation Solution Problem Understanding The problem asks whether the characters of a given string can be rearranged to form a palindrome. A palindrome is a string that reads the same forward and backward. Examples include "racecar" , "abba" , and "a" . The important detail is that we are not asked to actually generate the palindrome. We only need...

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LeetCode 1477 - Find Two Non-overlapping Sub-arrays Each With Target Sum

LeetCode Problem 1477 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Dynamic Programming, Sliding Window Solution Problem Understanding The problem asks us to find two different subarrays inside the given array such that: Each subarray has a sum exactly equal to target The two subarrays do not overlap The total length of the two chosen subarrays is as small as possible A subarray is a contiguous portion of the...

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LeetCode 1656 - Design an Ordered Stream

LeetCode Problem 1656 Difficulty: 🟢 Easy Topics: Array, Hash Table, Design, Data Stream Solution Problem Understanding This problem asks us to simulate a data stream where each element arrives with a unique ID in arbitrary order. Each element consists of an integer idKey (between 1 and n ) and a string value . The goal is to design a stream that, after every insertion, returns the largest contiguous chunk of...

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LeetCode 854 - K-Similar Strings

LeetCode Problem 854 Difficulty: 🔴 Hard Topics: Hash Table, String, Breadth-First Search Solution Problem Understanding The problem asks us to determine the minimum number of swaps needed to transform one string into another, where both strings are anagrams of each other. A single operation consists of swapping any two characters in s1 . The goal is to make s1 exactly equal to s2 using as few swaps as possible. Because...

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LeetCode 771 - Jewels and Stones

LeetCode Problem 771 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding In this problem, we are given two strings, jewels and stones . The string jewels represents all stone types that are considered jewels. Each character is a unique jewel type. For example, if jewels = "aA" , then both lowercase 'a' and uppercase 'A' are jewel types. The string stones represents all stones we currently own. Every...

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LeetCode 2025 - Maximum Number of Ways to Partition an Array

LeetCode Problem 2025 Difficulty: 🔴 Hard Topics: Array, Hash Table, Counting, Enumeration, Prefix Sum Solution Problem Understanding The problem asks us to count how many valid partition points exist in an array after optionally changing at most one element to a given value k . A partition is defined by a pivot index pivot such that: 1 <= pivot < n The sum of elements on the left side equals...

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LeetCode 947 - Most Stones Removed with Same Row or Column

LeetCode Problem 947 Difficulty: 🟡 Medium Topics: Hash Table, Depth-First Search, Union-Find, Graph Theory Solution Problem Understanding The problem gives us a collection of stones placed on a 2D grid. Each stone occupies a unique coordinate (x, y) . A stone can be removed only if there is at least one other stone that shares either the same row or the same column. The goal is to determine the maximum...

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LeetCode 409 - Longest Palindrome

LeetCode Problem 409 Difficulty: 🟢 Easy Topics: Hash Table, String, Greedy Solution Problem Understanding The problem asks us to find the length of the longest palindrome that can be constructed using the characters of a given string s . A palindrome is a string that reads the same forwards and backwards. The input string s contains only lowercase and uppercase English letters, and letter case matters, so 'A' and 'a'...

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LeetCode 1110 - Delete Nodes And Return Forest

LeetCode Problem 1110 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Binary Tree Solution LeetCode 1110 - Delete Nodes And Return Forest Problem Understanding This problem gives us the root of a binary tree and a list of node values that must be deleted from the tree. Every node value in the tree is unique, which is important because it means we can identify nodes directly by value...

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LeetCode 1366 - Rank Teams by Votes

LeetCode Problem 1366 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Counting Solution Problem Understanding The problem describes a voting based ranking system where every voter ranks all teams from best to worst. Each vote is represented as a string, where the character at index 0 is the top ranked team, the character at index 1 is the second ranked team, and so on. The final ranking is determined...

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LeetCode 914 - X of a Kind in a Deck of Cards

LeetCode Problem 914 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting, Number Theory Solution Problem Understanding The problem asks whether it is possible to partition a deck of cards, represented as an integer array deck , into groups such that each group contains exactly x cards, all of which have the same integer value, and x is greater than 1. Essentially, we need to find a number x that...

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LeetCode 1036 - Escape a Large Maze

LeetCode Problem 1036 Difficulty: 🔴 Hard Topics: Array, Hash Table, Depth-First Search, Breadth-First Search Solution Problem Understanding The problem gives us a massive 1,000,000 x 1,000,000 grid. Each cell is identified by coordinates (x, y) , and movement is allowed in four directions: up, down, left, and right. Some cells are blocked, meaning we cannot step onto them. We are given three inputs: blocked , a list of blocked coordinates...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 648 - Replace Words

LeetCode Problem 648 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding The problem asks us to replace words in a sentence using a set of predefined root words . A root is a shorter word that can serve as a prefix for a longer derivative word. Whenever a word in the sentence starts with one of the roots from the dictionary, we replace that word with...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2963 - Count the Number of Good Partitions

LeetCode Problem 2963 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Combinatorics Solution Problem Understanding The problem asks us to count the number of ways we can partition a given array nums of positive integers into contiguous subarrays such that no number appears in more than one subarray. Each partition must maintain the original order of elements, and two partitions are considered different if the subarrays are split at different...

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LeetCode 1452 - People Whose List of Favorite Companies Is Not a Subset of Another List

LeetCode Problem 1452 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem requires identifying all people in a group whose list of favorite companies is not a subset of any other person's list . Each person has a list of companies they like, and we want the indices of people whose lists are unique in the sense that no other list completely contains theirs. The input...

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LeetCode 170 - Two Sum III - Data structure design

LeetCode Problem 170 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Design, Data Stream Solution Problem Understanding The problem asks us to design a data structure that supports two operations efficiently over a stream of integers. The first operation, add(number) , inserts a number into the data structure. Numbers may appear multiple times, so duplicates must be handled correctly. The second operation, find(value) , checks whether there exists any...

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LeetCode 911 - Online Election

LeetCode Problem 911 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design Solution Problem Understanding The problem is asking us to design a data structure that can efficiently answer queries about the leader of an election at any given time. We are given two integer arrays, persons and times . The persons array represents the sequence of votes, where persons[i] is the candidate who received the i th vote....

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LeetCode 1258 - Synonymous Sentences

LeetCode Problem 1258 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Backtracking, Sort, Union-Find Solution Problem Understanding This problem asks us to generate all possible synonymous variations of a given sentence based on a list of equivalent word pairs. Each synonym pair indicates that the two words can replace each other interchangeably. The output must include all possible sentences created by substituting words with their synonyms and must be sorted...

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LeetCode 1282 - Group the People Given the Group Size They Belong To

LeetCode Problem 1282 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy Solution Problem Understanding The problem gives us an array called groupSizes , where each index represents a person and each value represents the size of the group that person must belong to. Every person has a unique ID ranging from 0 to n - 1 , where n is the length of the array. For any person i ,...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1740 - Find Distance in a Binary Tree

LeetCode Problem 1740 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding This problem asks us to determine the distance between two nodes in a binary tree, given their values p and q . The distance is defined as the number of edges in the shortest path connecting the two nodes. The input is the root of a binary tree and two integers...

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LeetCode 1519 - Number of Nodes in the Sub-Tree With the Same Label

LeetCode Problem 1519 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Counting Solution Problem Understanding The problem gives us a tree with n nodes numbered from 0 to n - 1 . The tree is represented by an array of edges , where each edge connects two nodes ai and bi . Each node also has a label, represented as a string labels , where labels[i] is...

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LeetCode 2007 - Find Original Array From Doubled Array

LeetCode Problem 2007 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting Solution Problem Understanding The problem gives us an array called changed , which was supposedly created from another array called original . The transformation process works like this: Take every number in original . Append its doubled value, meaning 2 * x . Shuffle all the numbers together. For example, if: original = [1, 3, 4] then the...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 350 - Intersection of Two Arrays II

LeetCode Problem 350 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Binary Search, Sorting Solution Problem Understanding The problem asks us to find the intersection of two integer arrays, but with an important detail: duplicates matter. If a number appears multiple times in both arrays, it must appear in the result as many times as it appears in both. For example, if 2 appears twice in nums1 and three...

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LeetCode 522 - Longest Uncommon Subsequence II

LeetCode Problem 522 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Sorting Solution Problem Understanding The problem asks us to find the length of the longest string in the array that is not a subsequence of any other string in the same array. A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters. For example, "abc" is...

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LeetCode 1074 - Number of Submatrices That Sum to Target

LeetCode Problem 1074 Difficulty: 🔴 Hard Topics: Array, Hash Table, Matrix, Prefix Sum Solution Problem Understanding The problem asks us to count how many non-empty rectangular submatrices inside a 2D matrix have a sum equal to a given target . A submatrix is any contiguous rectangular region within the matrix. If we choose two row boundaries and two column boundaries, every element inside that rectangle belongs to the submatrix. The...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 447 - Number of Boomerangs

LeetCode Problem 447 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math Solution Problem Understanding The problem asks us to count the number of valid boomerangs among a set of distinct 2D points. A boomerang is defined as a tuple (i, j, k) where the distance between point i and point j is equal to the distance between point i and point k . The important detail is that the order...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 888 - Fair Candy Swap

LeetCode Problem 888 Difficulty: 🟢 Easy Topics: Array, Hash Table, Binary Search, Sorting Solution LeetCode 888 - Fair Candy Swap Problem Understanding The problem gives us two arrays, aliceSizes and bobSizes , representing the candy boxes owned by Alice and Bob. Each element in the arrays is the number of candies in a particular box. Alice and Bob want to exchange exactly one box each so that, after the swap,...

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LeetCode 1257 - Smallest Common Region

LeetCode Problem 1257 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Tree, Depth-First Search, Breadth-First Search Solution Problem Understanding This problem describes a hierarchical relationship between geographic regions. Each list in regions represents a parent-child relationship where the first element is the parent region and every remaining element in the list is directly contained within that parent. For example, the list: ["North America", "United States", "Canada"] means that "North America"...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 398 - Random Pick Index

LeetCode Problem 398 Difficulty: 🟡 Medium Topics: Hash Table, Math, Reservoir Sampling, Randomized Solution Problem Understanding The problem gives us an integer array nums that may contain duplicate values. We must design a class that supports repeatedly selecting a random index for a given target value. The important requirement is that if the target appears multiple times, every valid index must have the same probability of being returned. For example,...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 839 - Similar String Groups

LeetCode Problem 839 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find Solution Problem Understanding The problem asks us to determine the number of similarity groups within a list of strings. Two strings are defined as similar if they are either identical or if we can swap exactly two letters in one string to make it equal to the other. For example, "tars" and "rats" are...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1488 - Avoid Flood in The City

LeetCode Problem 1488 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Greedy, Heap (Priority Queue) Solution Problem Understanding The problem asks us to simulate rainfall over a series of lakes and prevent flooding. Each day is represented by an integer in the rains array. If rains[i] > 0 , it means lake rains[i] receives rain and becomes full. If it is already full, it floods, which we must avoid....

Practice › LeetCode › LeetCode 08xx ›

LeetCode 811 - Subdomain Visit Count

LeetCode Problem 811 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding In this problem, we are given a list of strings where each string represents a visit count paired with a domain name. Each input entry has the form: "count domain" For example: "9001 discuss.leetcode.com" means the domain discuss.leetcode.com was visited 9001 times. The important detail is that visiting a domain also counts as visiting all...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 299 - Bulls and Cows

LeetCode Problem 299 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives two strings, secret and guess , representing two numbers of equal length. We need to compare them and return a hint in the format "xAyB" . A "bull" is a digit that matches in both value and position. For example, if secret[i] == guess[i] , then that digit contributes one bull. A "cow"...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 249 - Group Shifted Strings

LeetCode Problem 249 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to group strings that belong to the same cyclic shifting sequence. Two strings belong to the same group if one can be transformed into the other by repeatedly shifting every character forward or backward in the alphabet, with wraparound between 'z' and 'a' . For example, "abc" becomes "bcd" after one right...

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LeetCode 981 - Time Based Key-Value Store

LeetCode Problem 981 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Design Solution Problem Understanding The problem asks us to design a data structure that behaves like a key-value store, but with an important twist: the same key can have multiple values over time. Every value is associated with a timestamp, and later queries must return the most recent value that existed at or before a requested timestamp. In...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 36 - Valid Sudoku

LeetCode Problem 36 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution Problem Understanding This problem asks us to validate whether a partially filled Sudoku board follows the core Sudoku rules. The board is always a fixed 9 x 9 grid, and each cell contains either a digit from '1' to '9' or the character '.' , which represents an empty cell. A valid Sudoku configuration must satisfy three independent...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 588 - Design In-Memory File System

LeetCode Problem 588 Difficulty: 🔴 Hard Topics: Hash Table, String, Design, Trie, Sorting Solution Problem Understanding The problem asks us to design an in-memory file system that simulates basic file system operations without interacting with the real filesystem. We need to implement a FileSystem class that supports creating directories, adding and reading file content, and listing files or directories in a given path. The input consists of paths, which are...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1640 - Check Array Formation Through Concatenation

LeetCode Problem 1640 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 1640 - Check Array Formation Through Concatenation Problem Understanding This problem gives us two inputs: arr , a one-dimensional array of distinct integers pieces , a collection of smaller integer arrays The goal is to determine whether we can recreate arr by concatenating the arrays inside pieces in any order. The important restriction is that we are not...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 491 - Non-decreasing Subsequences

LeetCode Problem 491 Difficulty: 🟡 Medium Topics: Array, Hash Table, Backtracking, Bit Manipulation Solution LeetCode 491 - Non-decreasing Subsequences Problem Understanding The problem gives us an integer array nums and asks us to return every distinct subsequence that is non-decreasing and has length at least two. A subsequence is formed by deleting zero or more elements from the array without changing the relative order of the remaining elements. Unlike subarrays,...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1079 - Letter Tile Possibilities

LeetCode Problem 1079 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking, Counting Solution Problem Understanding The problem asks us to count all possible non-empty sequences that can be formed from a set of letter tiles, where each tile has a single uppercase letter. The input is a string tiles representing the letters available on the tiles. The output is an integer representing the total number of distinct sequences that can...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1711 - Count Good Meals

LeetCode Problem 1711 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding The problem asks us to count the number of good meals that can be formed from a given list of food items. A good meal is defined as a pair of two different items whose combined deliciousness is a power of two . The input is an array of integers, deliciousness , where each element represents the...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1512 - Number of Good Pairs

LeetCode Problem 1512 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting Solution LeetCode 1512 - Number of Good Pairs Problem Understanding The problem gives us an integer array nums and asks us to count how many pairs of indices (i, j) satisfy two conditions: nums[i] == nums[j] i < j Such pairs are called "good pairs". In simpler terms, we need to count how many times the same value...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 314 - Binary Tree Vertical Order Traversal

LeetCode Problem 314 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Sorting, Binary Tree Solution Problem Understanding The problem asks us to group the nodes of a binary tree by their vertical columns and return those groups from left to right. Each node in the tree belongs to a conceptual coordinate system: The root starts at column 0 Moving to the left child decreases the column index...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 770 - Basic Calculator IV

LeetCode Problem 770 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Stack, Recursion Solution Problem Understanding The problem requires parsing and evaluating a mathematical expression that includes integers, variables, addition, subtraction, multiplication, and parentheses, while also applying a substitution map for certain variables. Essentially, we are building a symbolic algebra engine that simplifies expressions according to standard arithmetic rules. The input consists of a string expression that may contain integers,...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1153 - String Transforms Into Another String

LeetCode Problem 1153 Difficulty: 🔴 Hard Topics: Hash Table, String, Graph Theory Solution Problem Understanding The problem gives us two strings, str1 and str2 , which are guaranteed to have the same length. We want to determine whether it is possible to transform str1 into str2 using a sequence of character conversion operations. A single conversion operation works as follows: Choose one character in str1 Replace every occurrence of that...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1497 - Check If Array Pairs Are Divisible by k

LeetCode Problem 1497 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution I can do that, but the full guide at the level of detail you requested, including complete prose explanations, worked traces, Python and Go implementations, comprehensive test cases, and edge-case analysis, will be quite long and may exceed a single message limit. To preserve quality and avoid truncation, I will split it into 2 parts : Part 1...

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LeetCode 2013 - Detect Squares

LeetCode Problem 2013 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Counting, Data Stream Solution Problem Understanding The problem presents a stream of points in a 2D plane and asks us to design a data structure that supports two operations efficiently: adding points and counting squares. The add operation inserts a point into the data structure, and the count operation calculates the number of axis-aligned squares that can be formed...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 767 - Reorganize String

LeetCode Problem 767 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The problem asks us to rearrange the characters of a given string s such that no two adjacent characters are the same. The input is a string of lowercase English letters with a length between 1 and 500. The output should be either a valid rearrangement that satisfies the adjacency constraint...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 711 - Number of Distinct Islands II

LeetCode Problem 711 Difficulty: 🔴 Hard Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Sorting, Matrix, Hash Function Solution Problem Understanding In this problem, we are given a binary matrix where each cell contains either 0 or 1 . A value of 1 represents land, while 0 represents water. An island is formed by connecting adjacent land cells in the four cardinal directions: up, down, left, and right. The...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 957 - Prison Cells After N Days

LeetCode Problem 957 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Bit Manipulation Solution Problem Understanding The problem presents a simulation scenario with a row of exactly 8 prison cells, where each cell is either occupied (1) or vacant (0). Each day, the state of a cell changes based on the states of its immediate neighbors. Specifically, a cell becomes occupied if both of its neighbors are in the same...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1338 - Reduce Array Size to The Half

LeetCode Problem 1338 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue) Solution Here is a complete, detailed technical solution guide for LeetCode 1338, following your requested formatting and structure. Problem Understanding The problem asks us to reduce the size of an integer array arr by removing all occurrences of a selected set of integers. Specifically, we want to find the minimum size of such a set...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 781 - Rabbits in Forest

LeetCode Problem 781 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Greedy Solution Problem Understanding In this problem, every rabbit tells us how many other rabbits share its color. The input array answers contains these responses. If a rabbit says x , that means there are exactly x + 1 rabbits of that color group in total, including itself. The challenge is that we do not know which rabbits belong...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1410 - HTML Entity Parser

LeetCode Problem 1410 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Here is a complete, detailed technical guide for LeetCode 1410 following your formatting and style requirements. Problem Understanding The problem asks us to implement an HTML entity parser . We are given a string text which may contain HTML entities such as &quot; for " or &amp; for & . The goal is to replace all valid entities in...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1737 - Change Minimum Characters to Satisfy One of Three Conditions

LeetCode Problem 1737 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting, Prefix Sum Solution Problem Understanding The problem gives us two lowercase English strings, a and b . In a single operation, we may change any character in either string into any other lowercase English letter. Our task is to determine the minimum number of character changes required so that at least one of the following conditions becomes true: Every...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1650 - Lowest Common Ancestor of a Binary Tree III

LeetCode Problem 1650 Difficulty: 🟡 Medium Topics: Hash Table, Two Pointers, Tree, Binary Tree Solution Problem Understanding This problem asks us to find the lowest common ancestor, usually abbreviated as LCA, of two nodes in a binary tree. Unlike the classic version of the problem, every node contains a direct pointer to its parent. That extra parent reference changes the way we can solve the problem efficiently. The input gives...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 710 - Random Pick with Blacklist

LeetCode Problem 710 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Binary Search, Sorting, Randomized Solution Problem Understanding The problem asks us to design a data structure that can repeatedly return a random integer from the range [0, n - 1] , while excluding all integers that appear in a blacklist. Every valid number must have exactly the same probability of being chosen. The challenge is not simply filtering numbers....

Practice › LeetCode › LeetCode 09xx ›

LeetCode 924 - Minimize Malware Spread

LeetCode Problem 924 Difficulty: 🔴 Hard Topics: Array, Hash Table, Depth-First Search, Breadth-First Search, Union-Find, Graph Theory Solution Problem Understanding In this problem, we are given an undirected graph represented as an adjacency matrix. Each node represents a computer in a network, and an edge between two nodes means those computers are directly connected. Some subset of nodes is initially infected with malware. Malware spreads through connections, meaning that if...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 560 - Subarray Sum Equals K

LeetCode Problem 560 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem asks us to count how many contiguous, non-empty subarrays in an integer array nums have a sum equal to a target value k . A subarray is different from a subsequence. Since the elements must remain contiguous, we only consider segments of the array that occupy consecutive positions. For example, in [1,2,3] ,...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 652 - Find Duplicate Subtrees

LeetCode Problem 652 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem gives us the root of a binary tree and asks us to identify all duplicate subtrees inside it. A subtree is defined as any node together with all of its descendants. Two subtrees are considered duplicates if they have exactly the same structure and every corresponding node contains the same value....

Practice › LeetCode › LeetCode 03xx ›

LeetCode 336 - Palindrome Pairs

LeetCode Problem 336 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Trie, Hash Function Solution Problem Understanding The problem gives us a list of unique strings called words . We must find every ordered pair of indices (i, j) such that: i != j concatenating words[i] + words[j] forms a palindrome A palindrome is a string that reads the same forward and backward. The important detail is that the pairs...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1772 - Sort Features by Popularity

LeetCode Problem 1772 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding This problem asks us to determine the popularity of product features based on user survey responses. We are given a list of features where each element is a single-word feature name, and a list of responses where each element is a string of space-separated words that users reported liking. The popularity of a feature is...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1169 - Invalid Transactions

LeetCode Problem 1169 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem asks us to identify transactions that are potentially invalid based on two rules. First, any transaction with an amount greater than $1000 is invalid. Second, any transaction is invalid if there exists another transaction with the same name that occurs within 60 minutes but in a different city. The input is a list...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1048 - Longest String Chain

LeetCode Problem 1048 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Dynamic Programming, Sorting Solution That is a long, structured technical guide that will exceed a practical single-message limit if done comprehensively with all requested sections, detailed walkthroughs, Python and Go solutions, worked examples, full test suite, and edge case analysis. I can provide it in full, following your exact formatting rules. I will include: Problem Understanding Brute...

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LeetCode 1054 - Distant Barcodes

LeetCode Problem 1054 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The problem is asking us to rearrange a list of barcodes so that no two adjacent barcodes are the same. The input is an array barcodes of integers where each integer represents a type of barcode. The output should be a rearranged array that satisfies the adjacency constraint. The constraints specify...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1775 - Equal Sum Arrays With Minimum Number of Operations

LeetCode Problem 1775 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding The problem gives us two integer arrays, nums1 and nums2 , where every element is between 1 and 6 . In one operation, we may choose any element from either array and change it to any value from 1 to 6 . Our goal is to make the total sum of both arrays equal while...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 424 - Longest Repeating Character Replacement

LeetCode Problem 424 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem gives us a string s consisting only of uppercase English letters and an integer k . We are allowed to perform at most k replacement operations. In one operation, we can change any character in the string into any other uppercase English character. The goal is to find the length of the longest...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 859 - Buddy Strings

LeetCode Problem 859 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks whether it is possible to make two strings equal by performing exactly one swap operation on the first string s . A swap operation means choosing two different indices i and j in s and exchanging the characters at those positions. After performing that single swap, the resulting string must match goal . The...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1261 - Find Elements in a Contaminated Binary Tree

LeetCode Problem 1261 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Design, Binary Tree Solution Problem Understanding The problem gives us a binary tree where every node value has been contaminated and replaced with -1 . However, we know the exact rule that originally generated the values in the tree. The original tree followed these rules: The root node always had value 0 If a node had...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1695 - Maximum Erasure Value

LeetCode Problem 1695 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window Solution LeetCode 1695, Maximum Erasure Value Problem Understanding This problem asks us to find a contiguous subarray that contains only unique elements, meaning no value appears more than once inside that subarray. Among all such valid subarrays, we must return the maximum possible sum of its elements. The input is an array nums consisting of positive integers. Since...

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LeetCode 1657 - Determine if Two Strings Are Close

LeetCode Problem 1657 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting, Counting Solution Problem Understanding The problem asks whether two strings, word1 and word2 , are "close" according to two allowed operations. Operation 1 allows swapping any two existing characters in the string, which means the relative order of characters is flexible. Operation 2 allows transforming all occurrences of one character into another existing character, and vice versa, which effectively...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1830 - Minimum Number of Operations to Make String Sorted

LeetCode Problem 1830 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Combinatorics, Counting Solution Problem Understanding The operation described in the problem is exactly the process of generating the previous lexicographical permutation of a string. Starting from the current string, each operation transforms it into the lexicographically largest string that is still smaller than the current one. The goal is to determine how many such operations are required before the...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 244 - Shortest Word Distance II

LeetCode Problem 244 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Design Solution LeetCode 244 - Shortest Word Distance II Problem Understanding This problem asks us to design a reusable data structure that can efficiently answer repeated shortest-distance queries between words in a fixed list of strings. We are given an array of words, wordsDict , during initialization. After that, we must repeatedly answer queries asking: What is...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1346 - Check If N and Its Double Exist

LeetCode Problem 1346 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Binary Search, Sorting Solution Problem Understanding The problem gives an integer array arr and asks whether there exist two different indices i and j such that: arr[i] == 2 * arr[j] In simpler terms, we need to determine whether one number in the array is exactly double another number in the same array. The indices must be different,...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 17 - Letter Combinations of a Phone Number

LeetCode Problem 17 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution Problem Understanding The problem gives a string of digits where each digit is between 2 and 9 . Each digit corresponds to a set of letters on a traditional phone keypad: 2 -> "abc" 3 -> "def" 4 -> "ghi" 5 -> "jkl" 6 -> "mno" 7 -> "pqrs" 8 -> "tuv" 9 -> "wxyz" We must generate...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 432 - All O`one Data Structure

LeetCode Problem 432 Difficulty: 🔴 Hard Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design a special data structure that stores string keys together with their occurrence counts. The structure must support incrementing and decrementing counts, while also being able to instantly retrieve one key with the maximum count and one key with the minimum count. Unlike ordinary frequency counting problems, this...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 884 - Uncommon Words from Two Sentences

LeetCode Problem 884 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution LeetCode 884, Uncommon Words from Two Sentences Problem Understanding This problem asks us to find all words that are considered "uncommon" between two sentences. Each sentence is made up of lowercase words separated by single spaces. A word is considered uncommon if it satisfies two conditions: It appears exactly once in one sentence. It does not appear at...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 128 - Longest Consecutive Sequence

LeetCode Problem 128 Difficulty: 🟡 Medium Topics: Array, Hash Table, Union-Find Solution Problem Understanding The problem asks us to find the length of the longest sequence of consecutive integers in an unsorted array. A consecutive sequence means numbers that appear one after another numerically, regardless of their position in the array. The numbers do not need to be adjacent in the input, and the array itself is not sorted. In...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 442 - Find All Duplicates in an Array

LeetCode Problem 442 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem gives us an integer array nums of length n , where every value is guaranteed to be in the range [1, n] . Each number appears either once or twice. Our task is to return all numbers that appear exactly twice. The important requirement is not just correctness, but efficiency. The solution must run...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 820 - Short Encoding of Words

LeetCode Problem 820 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding The problem asks us to find the length of the shortest reference string that can encode a list of words. A reference string is formed by concatenating some of the words with a '#' character at the end of each word. Importantly, a word can be represented by a substring of the reference string if...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1654 - Minimum Jumps to Reach Home

LeetCode Problem 1654 Difficulty: 🟡 Medium Topics: Array, Hash Table, Breadth-First Search Solution Problem Understanding This problem asks us to find the minimum number of jumps required for a bug to move from position 0 to position x on a one dimensional number line. The bug follows several movement rules: It may jump forward by exactly a units. It may jump backward by exactly b units. It may not perform...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1612 - Check If Two Expression Trees are Equivalent

LeetCode Problem 1612 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Binary Tree, Counting Solution Problem Understanding The problem asks us to determine whether two binary expression trees are equivalent, where equivalence is defined as the trees representing the same arithmetic expression, modulo the order of addition since addition is commutative. Each tree node contains either a '+' operator (for internal nodes) or a variable (for leaf nodes). The...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 939 - Minimum Area Rectangle

LeetCode Problem 939 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Geometry, Sorting Solution Problem Understanding The problem gives us a collection of distinct points on a 2D plane. Each point is represented as [x, y] , where x is the horizontal coordinate and y is the vertical coordinate. Our task is to determine the minimum possible area of an axis-aligned rectangle that can be formed using four of these...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1345 - Jump Game IV

LeetCode Problem 1345 Difficulty: 🔴 Hard Topics: Array, Hash Table, Breadth-First Search Solution Problem Understanding The problem is asking for the minimum number of steps required to reach the last index of an integer array, starting from the first index, given specific movement rules. You can move to the next index ( i + 1 ), the previous index ( i - 1 ), or any other index j where...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1790 - Check if One String Swap Can Make Strings Equal

LeetCode Problem 1790 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine if two strings of equal length, s1 and s2 , can be made identical by performing at most one string swap on exactly one of the strings . A string swap allows exchanging any two characters at different or same indices. The task is not to perform multiple swaps or...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 823 - Binary Trees With Factors

LeetCode Problem 823 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming, Sorting Solution Problem Understanding The problem asks us to determine the number of distinct binary trees that can be constructed from a given array of unique integers, where each integer is strictly greater than 1. The key constraint is that for every non-leaf node in the tree, its value must equal the product of its two children. Leaf...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1796 - Second Largest Digit in a String

LeetCode Problem 1796 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem gives us a string s that contains lowercase English letters and numerical digits. Our task is to find the second largest distinct digit that appears anywhere in the string. The important detail is that we care about distinct digits, not frequency. If the string contains "111222333" , the distinct digits are {1, 2, 3} ....

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1027 - Longest Arithmetic Subsequence

LeetCode Problem 1027 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Dynamic Programming Solution Your requested guide is very long and detailed, especially with all required sections, full prose explanations, worked traces, Python and Go implementations, complexity proofs, and extensive test coverage. To maintain quality and avoid truncation, I will provide the complete guide in a structured single response across consecutive parts, preserving all formatting requirements exactly. I’ll begin...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1418 - Display Table of Food Orders in a Restaurant

LeetCode Problem 1418 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Ordered Set Solution Problem Understanding The problem gives a list of restaurant orders. Each order contains three pieces of information: Customer name Table number Food item An order looks like this: [customerName, tableNumber, foodItem] The goal is to build a display table that summarizes how many times each food item was ordered at every table. The output format...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 874 - Walking Robot Simulation

LeetCode Problem 874 Difficulty: 🟡 Medium Topics: Array, Hash Table, Simulation Solution Problem Understanding The problem describes a robot moving on an infinite two dimensional grid. The robot begins at coordinate (0, 0) and initially faces north, which corresponds to the positive Y direction. The robot processes a sequence of commands, where each command either changes its direction or moves it forward. There are three kinds of commands: -2 means...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 166 - Fraction to Recurring Decimal

LeetCode Problem 166 Difficulty: 🟡 Medium Topics: Hash Table, Math, String Solution Problem Understanding This problem asks us to convert a fraction, represented by an integer numerator and an integer denominator , into its decimal string representation. If the decimal expansion contains a repeating fractional sequence, we must enclose the repeating part in parentheses. For example, the fraction 1/2 produces "0.5" because it terminates. The fraction 1/3 produces "0.(3)" because...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 41 - First Missing Positive

LeetCode Problem 41 Difficulty: 🔴 Hard Topics: Array, Hash Table Solution Problem Understanding The problem asks us to find the smallest positive integer that does not appear in an unsorted integer array. The key detail is that we only care about positive integers starting from 1 . Negative numbers, zero, and very large positive numbers are irrelevant unless they affect the placement of smaller positive integers. For example: In [1,2,0]...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1429 - First Unique Number

LeetCode Problem 1429 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Queue, Data Stream Solution Problem Understanding The problem requires designing a data structure that can efficiently maintain and retrieve the first unique integer in a queue of numbers. You are given an initial list of integers, and after that, the data structure should support two operations efficiently: retrieving the first unique number ( showFirstUnique ) and adding a new...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 383 - Ransom Note

LeetCode Problem 383 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks whether it is possible to construct the string ransomNote using characters taken from the string magazine . Each character in magazine can only be used once, which means character frequency matters. If ransomNote requires more occurrences of a letter than magazine contains, the answer must be false . In other words, we need...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 353 - Design Snake Game

LeetCode Problem 353 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Queue, Simulation Solution Problem Understanding The problem asks us to design a mutable data structure that simulates the classic Snake game. The game is played on a rectangular grid with dimensions height x width . The snake starts at the top-left corner (0, 0) and initially has length 1 . The game receives a predefined sequence of food positions....

Practice › LeetCode › LeetCode 06xx ›

LeetCode 690 - Employee Importance

LeetCode Problem 690 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Breadth-First Search Solution Problem Understanding The problem provides a collection of employees where each employee contains three pieces of information: A unique employee ID An integer importance value A list of IDs representing their direct subordinates We are also given the ID of one employee, and we must calculate the total importance for that employee and everyone...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 348 - Design Tic-Tac-Toe

LeetCode Problem 348 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Matrix, Simulation Solution Problem Understanding The problem asks us to design a data structure that simulates an n x n Tic-Tac-Toe game between two players. We need to implement a class that supports two operations: Initializing a game board of size n Processing moves one at a time and immediately determining whether the current move causes a player to...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1577 - Number of Ways Where Square of Number Is Equal to Product of Two Numbers

LeetCode Problem 1577 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Two Pointers Solution LeetCode 1577, Number of Ways Where Square of Number Is Equal to Product of Two Numbers Problem Understanding The problem gives us two integer arrays, nums1 and nums2 . We must count the number of valid triplets that satisfy one of two mathematical relationships. A Type 1 triplet exists when: We pick one element from nums1...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1719 - Number Of Ways To Reconstruct A Tree

LeetCode Problem 1719 Difficulty: 🔴 Hard Topics: Array, Hash Table, Tree, Graph Theory, Simulation Solution Problem Understanding The problem provides an array of pairs , where each pair [xi, yi] indicates that xi is either an ancestor of yi or yi is an ancestor of xi in a rooted tree. The key challenge is to determine how many different rooted trees satisfy all given pairs. We are not limited to...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 822 - Card Flipping Game

LeetCode Problem 822 Difficulty: 🟡 Medium Topics: Array, Hash Table Solution Problem Understanding In this problem, we are given a collection of cards where each card has two numbers, one on the front and one on the back. Initially, every card is placed with its front side facing upward. We are allowed to flip any subset of cards, including flipping none of them or flipping all of them. After choosing...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1684 - Count the Number of Consistent Strings

LeetCode Problem 1684 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Bit Manipulation, Counting Solution Problem Understanding This problem asks us to determine how many strings in the words array are consistent with a given set of allowed characters. You are given two inputs: allowed , a string containing distinct lowercase English letters. words , an array of lowercase strings. A word is considered consistent if every character in that...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1189 - Maximum Number of Balloons

LeetCode Problem 1189 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine the maximum number of times the word "balloon" can be formed using the characters from a given string text . Each character in text can be used at most once per occurrence of the word. The input is a string consisting of only lowercase English letters, with length up to...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 242 - Valid Anagram

LeetCode Problem 242 Difficulty: 🟢 Easy Topics: Hash Table, String, Sorting Solution Problem Understanding The problem asks us to determine whether two strings are anagrams of each other. Two strings are considered anagrams if they contain exactly the same characters with exactly the same frequencies , but possibly in a different order. In other words, one string can be rearranged to form the other. For example, "anagram" and "nagaram" are...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 635 - Design Log Storage System

LeetCode Problem 635 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Ordered Set Solution Problem Understanding The problem is asking us to design a log storage system that supports two primary operations: storing logs with unique IDs and timestamps, and retrieving logs based on a timestamp range and granularity. Each timestamp is represented as a string in the format YYYY:MM:DD:HH:MM:SS , which is zero-padded and guarantees lexicographic ordering aligns with...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1156 - Swap For Longest Repeated Character Substring

LeetCode Problem 1156 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to determine the length of the longest substring in a given string text where all characters are the same, and we are allowed to swap exactly two characters in the string. The swap can be between any two characters, not necessarily adjacent, but we are limited to a single swap per...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1797 - Design Authentication Manager

LeetCode Problem 1797 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design an authentication system that manages tokens with expiration times. Each token is valid for a fixed timeToLive seconds starting from the moment it is generated or renewed. The system must support three operations: generating a new token, renewing an existing unexpired token, and counting the number of...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1676 - Lowest Common Ancestor of a Binary Tree IV

LeetCode Problem 1676 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Binary Tree Solution Problem Understanding This problem asks us to find the Lowest Common Ancestor, or LCA, of multiple nodes in a binary tree. Unlike the classic LCA problem that involves only two nodes, this variation generalizes the definition to any number of target nodes. The input consists of: root , the root node of a binary tree...

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LeetCode 1763 - Longest Nice Substring

LeetCode Problem 1763 Difficulty: 🟢 Easy Topics: Hash Table, String, Divide and Conquer, Bit Manipulation, Sliding Window Solution Problem Understanding The problem asks us to find the longest "nice" substring within a given string s . A substring is considered nice if for every letter it contains, both its lowercase and uppercase forms appear. For example, "aAa" is nice because it contains 'a' and 'A' . Conversely, "abA" is not...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2981 - Find Longest Special Substring That Occurs Thrice I

LeetCode Problem 2981 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Sliding Window, Counting Solution Problem Understanding The problem asks us to find the longest special substring that appears at least three times in the given string s . A substring is considered special if it consists entirely of the same character . This means strings like "aaa" , "bb" , and "z" are valid special substrings, while "ab"...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 889 - Construct Binary Tree from Preorder and Postorder Traversal

LeetCode Problem 889 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Tree, Binary Tree Solution Problem Understanding The problem gives us two traversal orders of the same binary tree: preorder , which visits nodes in the order: root → left subtree → right subtree postorder , which visits nodes in the order: left subtree → right subtree → root Our task is to reconstruct and return the binary...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 139 - Word Break

LeetCode Problem 139 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Dynamic Programming, Trie, Memoization Solution Problem Understanding The problem asks us to determine whether a given string s can be broken into a sequence of valid words from a given dictionary wordDict . Essentially, we need to check if we can insert spaces into s such that every substring separated by these spaces exists in the dictionary. The input...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 721 - Accounts Merge

LeetCode Problem 721 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find, Sorting Solution Problem Understanding This problem asks us to merge user accounts based on shared email addresses. Each account is represented as a list of strings. The first string is the user's name, and every remaining string is an email address associated with that account. The important detail is that two accounts belong to...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1123 - Lowest Common Ancestor of Deepest Leaves

LeetCode Problem 1123 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding This problem asks us to find the lowest common ancestor, abbreviated as LCA, of all the deepest leaf nodes in a binary tree. A leaf node is any node with no children. The depth of the root is 0 , and every level downward increases the depth by 1 . The...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1655 - Distribute Repeating Integers

LeetCode Problem 1655 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Backtracking, Bit Manipulation, Counting, Bitmask Solution Problem Understanding This problem asks whether we can satisfy a set of customer requests using repeated integers from the array nums . Each customer wants a certain quantity of numbers, given by quantity[i] . The important restriction is that every number given to a single customer must be identical. A customer asking...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1681 - Minimum Incompatibility

LeetCode Problem 1681 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Bit Manipulation, Bitmask Solution Problem Understanding The problem is asking us to partition an integer array nums into exactly k subsets of equal size, such that no subset contains duplicate elements. The incompatibility of a subset is defined as the difference between its maximum and minimum values. Our goal is to find the minimum possible sum of these...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 599 - Minimum Index Sum of Two Lists

LeetCode Problem 599 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem is asking us to find the common elements between two lists of strings, but not just any common elements. We want the ones where the sum of their indices in both lists is the smallest. In other words, if a string appears at index i in list1 and index j in list2 , we...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1100 - Find K-Length Substrings With No Repeated Characters

LeetCode Problem 1100 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of length k contain only unique characters. A substring is a contiguous portion of the string. For every possible substring of length k , we must determine whether all characters inside it are distinct. If they are, we count that substring toward the final answer. For...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2032 - Two Out of Three

LeetCode Problem 2032 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation Solution Problem Understanding The problem asks us to identify all distinct integers that appear in at least two out of three given integer arrays . The input consists of three arrays nums1 , nums2 , and nums3 , each containing integers in the range [1, 100] . The output is an array of integers that occur in at...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2229 - Check if an Array Is Consecutive

LeetCode Problem 2229 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem asks us to determine whether an integer array forms a perfect sequence of consecutive numbers without gaps or duplicates. More formally, if the array has length n and its minimum value is x , then the array is considered consecutive if it contains every integer in the range: $$[x, x + n - 1]$$...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1674 - Minimum Moves to Make Array Complementary

LeetCode Problem 1674 Difficulty: 🟡 Medium Topics: Array, Hash Table, Prefix Sum Solution Problem Understanding The problem requires us to transform an array nums of even length n into a complementary array . An array is complementary if for every index i , the sum of nums[i] + nums[n - 1 - i] is the same constant value. Each element in nums can be changed to any integer in the...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1562 - Find Latest Group of Size M

LeetCode Problem 1562 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Simulation Solution Edit LeetCode 1562 - Find Latest Group of Size M Problem Understanding This problem gives us a permutation array arr containing integers from 1 to n , where n is the size of the binary string we are building. Initially, the binary string contains only zeroes. At each step, we flip exactly one position from 0...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1133 - Largest Unique Number

LeetCode Problem 1133 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting Solution Problem Understanding The problem is asking us to identify the largest integer in an array that occurs exactly once. In other words, among all integers that are unique (appear only one time), we need to find the maximum. If no such integer exists, the output should be -1 . The input nums is a list of integers, each...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 500 - Keyboard Row

LeetCode Problem 500 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem asks us to determine which words in a given list can be typed using letters from only one row of an American keyboard. The input is an array of strings, words , where each string represents a word consisting only of English alphabet characters. The task is to return only those words where every...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 792 - Number of Matching Subsequences

LeetCode Problem 792 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Binary Search, Dynamic Programming, Trie, Sorting Solution Problem Understanding The problem asks us to count how many words in a given list are subsequences of a string s . A subsequence is formed by deleting zero or more characters from the string without changing the order of the remaining characters. For instance, "ace" is a subsequence of "abcde" because...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1443 - Minimum Time to Collect All Apples in a Tree

LeetCode Problem 1443 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search Solution Problem Understanding The problem is asking for the minimum time required to collect all apples in a tree, where each edge traversal takes 1 second. You start and end at vertex 0. The tree is given as an undirected graph with n vertices, described using an edges array. Each edge connects two vertices ai and...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 387 - First Unique Character in a String

LeetCode Problem 387 Difficulty: 🟢 Easy Topics: Hash Table, String, Queue, Counting Solution Problem Understanding The problem gives us a string s consisting only of lowercase English letters. We must find the index of the first character that appears exactly once in the entire string. If every character appears more than once, we return -1 . The key detail is that we are looking for the first non-repeating character based...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 805 - Split Array With Same Average

LeetCode Problem 805 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Dynamic Programming, Bit Manipulation, Bitmask Solution Problem Understanding The problem asks whether we can divide the given array nums into two non-empty groups such that both groups have the same average. Suppose the array is split into subsets A and B . Let: sum(A) be the sum of elements in A len(A) be the number of elements in A...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1072 - Flip Columns For Maximum Number of Equal Rows

LeetCode Problem 1072 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix Solution LeetCode 1072 - Flip Columns For Maximum Number of Equal Rows Problem Understanding The problem gives us an m x n binary matrix, where every element is either 0 or 1 . We are allowed to choose any set of columns and flip them. Flipping a column means changing every 0 in that column to 1 , and...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 745 - Prefix and Suffix Search

LeetCode Problem 745 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design a data structure that supports efficient searches based on both a prefix and a suffix. We are given an array of words, where each word has an implicit index based on its position in the array. The WordFilter class must support two operations: Initialization with the list of...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 996 - Number of Squareful Arrays

LeetCode Problem 996 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Dynamic Programming, Backtracking, Bit Manipulation, Bitmask Solution Problem Understanding The problem asks us to count all permutations of a given integer array nums such that the array is squareful , meaning that the sum of every pair of adjacent elements is a perfect square . In other words, for a permutation [a1, a2, a3, ..., an] , each a[i]...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1500 - Design a File Sharing System

LeetCode Problem 1500 Difficulty: 🟡 Medium Topics: Hash Table, Design, Sorting, Heap (Priority Queue), Data Stream Solution Problem Understanding This problem asks us to design a miniature peer-to-peer file sharing system. The file is divided into m chunks, where every chunk has an ID from 1 to m . Users can join the system while already owning some chunks, leave the system entirely, and request chunks from other users. The...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1600 - Throne Inheritance

LeetCode Problem 1600 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Design Solution Problem Understanding The problem asks us to design a data structure that simulates a royal inheritance system. There is a king at the top of the family tree, and over time people can be born or die. We must support three operations efficiently: Add a child to an existing person. Mark a person as dead. Return...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 352 - Data Stream as Disjoint Intervals

LeetCode Problem 352 Difficulty: 🔴 Hard Topics: Hash Table, Binary Search, Union-Find, Design, Data Stream, Ordered Set Solution LeetCode 352 - Data Stream as Disjoint Intervals Problem Understanding This problem asks us to process a stream of non-negative integers and continuously maintain a compact representation of all numbers seen so far. Instead of storing every individual number separately, we want to group consecutive numbers into disjoint intervals. An interval [start,...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1805 - Number of Different Integers in a String

LeetCode Problem 1805 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem requires us to process a string word that contains both lowercase English letters and digits. We are asked to extract all the sequences of digits as integers, ignoring any non-digit characters. After extracting these sequences, we need to count how many unique integers exist, considering that integers with leading zeros are equivalent to their non-zero...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 893 - Groups of Special-Equivalent Strings

LeetCode Problem 893 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem asks us to group strings based on a special equivalence property. Specifically, two strings are special-equivalent if you can swap characters at even indices among themselves and characters at odd indices among themselves any number of times to make the two strings identical. The input is an array of strings words , all...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 288 - Unique Word Abbreviation

LeetCode Problem 288 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding The problem asks us to design a data structure that determines whether a word's abbreviation is unique within a given dictionary. An abbreviation is defined using three parts: The first character The number of characters between the first and last character The last character For example: "dog" becomes "d1g" "internationalization" becomes "i18n" "it" remains "it"...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1146 - Snapshot Array

LeetCode Problem 1146 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design Solution Problem Understanding The problem asks us to implement a SnapshotArray , a data structure that behaves like an array but also allows taking snapshots of its state at any moment and retrieving the value of any element at a specific snapshot. Specifically, the data structure needs to support three operations: set(index, val) , snap() , and...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 480 - Sliding Window Median

LeetCode Problem 480 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sliding Window, Heap (Priority Queue) Solution Problem Understanding The problem asks us to compute the median for every contiguous subarray, or "window", of size k as that window slides from left to right across the input array. For each position of the window, we consider exactly k elements. We must determine the median of those elements and append it to...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 340 - Longest Substring with At Most K Distinct Characters

LeetCode Problem 340 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest contiguous substring in a given string s such that the substring contains at most k distinct characters. A substring is a continuous portion of the string. The important condition is not the total number of characters, but the number of unique character types inside...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 358 - Rearrange String k Distance Apart

LeetCode Problem 358 Difficulty: 🔴 Hard Topics: Hash Table, String, Greedy, Sorting, Heap (Priority Queue), Counting Solution LeetCode 358 - Rearrange String k Distance Apart Problem Understanding The problem gives us a string s and an integer k . We must rearrange the characters in the string so that identical characters are separated by at least k positions. The phrase "at least distance k " means that if the same...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 890 - Find and Replace Pattern

LeetCode Problem 890 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a list of words and a target pattern string. We need to determine which words follow the exact same structural character relationship as the pattern. A word matches the pattern if there exists a one to one mapping between characters in the pattern and characters in the word. This mapping must be...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 740 - Delete and Earn

LeetCode Problem 740 Difficulty: 🟡 Medium Topics: Array, Hash Table, Dynamic Programming Solution Problem Understanding The problem gives us an integer array nums , and we want to maximize the total number of points earned by repeatedly deleting elements. When we delete a number x , we gain x points. However, deleting x also forces the removal of every occurrence of x - 1 and x + 1 . This...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 760 - Find Anagram Mappings

LeetCode Problem 760 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution Problem Understanding The problem asks us to build an index mapping from nums1 to nums2 , where nums2 is guaranteed to be an anagram of nums1 . Since an anagram means the same elements appear in both arrays, but possibly in a different order, every value in nums1 must appear somewhere in nums2 . For each element nums1[i] ,...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1520 - Maximum Number of Non-Overlapping Substrings

LeetCode Problem 1520 Difficulty: 🔴 Hard Topics: Hash Table, String, Greedy, Sorting Solution LeetCode 1520 - Maximum Number of Non-Overlapping Substrings Problem Understanding This problem asks us to select as many non-overlapping substrings as possible from a given lowercase string s , while satisfying a strict containment rule. A substring is considered valid only if, whenever it contains a character c , it also contains every occurrence of c that...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 1 - Two Sum

LeetCode Problem 1 Difficulty: 🟢 Easy Topics: Array, Hash Table Solution LeetCode 1, Two Sum Problem Understanding The problem gives us an integer array called nums and another integer called target . Our task is to find two different elements in the array whose sum equals target , then return their indices. The important detail is that the output must contain indices, not the values themselves. For example, if the...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 202 - Happy Number

LeetCode Problem 202 Difficulty: 🟢 Easy Topics: Hash Table, Math, Two Pointers Solution Problem Understanding This problem asks us to determine whether a given positive integer n is a happy number . A happy number is defined through a repeated transformation process. Starting with the original number, we repeatedly replace it with the sum of the squares of its digits . Eventually, one of two things happens: The number becomes...

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LeetCode 13 - Roman to Integer

LeetCode Problem 13 Difficulty: 🟢 Easy Topics: Hash Table, Math, String Solution Problem Understanding The problem gives a Roman numeral string and asks us to convert it into its corresponding integer value. Roman numerals use seven symbols: Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 Most Roman numerals follow a simple additive rule. Symbols are written from largest to smallest, and we...

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LeetCode 205 - Isomorphic Strings

LeetCode Problem 205 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to determine whether two strings, s and t , are isomorphic . Two strings are considered isomorphic if there exists a one-to-one mapping between characters in s and characters in t such that replacing every character in s according to this mapping produces t . In simpler terms, each character in s must...

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LeetCode 3 - Longest Substring Without Repeating Characters

LeetCode Problem 3 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Longest Substring Without Repeating Characters Problem Understanding The problem gives a string s and asks for the length of the longest substring that contains no repeated characters. A substring is a continuous section of the string. This detail matters because characters must remain adjacent. For example, in "pwwkew" , the sequence "pwke" is not valid because those...

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LeetCode 208 - Implement Trie (Prefix Tree)

LeetCode Problem 208 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design and implement a Trie, also called a Prefix Tree. A Trie is a specialized tree structure for storing strings in a way that makes prefix-based operations very efficient. We need to implement three operations: insert(word) stores a word inside the Trie. search(word) checks whether the exact word exists in...

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LeetCode 1027: Longest Arithmetic Subsequence

Problem Restatement We are given an integer array nums . We need to return the length of the longest arithmetic subsequence in nums . A subsequence is arithmetic if the difference between consecutive elements is constant. The official constraints state that 2 <= nums.length <= 1000 and 0 <= nums[i] <= 500 . Input and Output Item Meaning Input Array nums Output Length of the longest arithmetic subsequence Subsequence Not...

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LeetCode 1079: Letter Tile Possibilities

Problem Restatement We have a set of tiles with letters on them. We need to return the number of possible non-empty sequences of letters using some or all of the tiles. Two sequences are the same if they use the same letters in the same order. The official constraints state that 1 <= tiles.length <= 7 and each tiles[i] is an uppercase English letter. Input and Output Item Meaning Input...

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LeetCode 1100: Find K-Length Substrings With No Repeated Characters

Problem Restatement We are given a string s and an integer k . Return the number of substrings of length k with no repeated characters. The official constraints state that 1 <= s.length <= 10^4 and 1 <= k <= 26 . Input and Output Item Meaning Input String s and integer k Output Count of length- k substrings with all distinct characters Function shape: def numKLenSubstrNoRepeats(s: str, k: int)...

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LeetCode 1054: Distant Barcodes

Problem Restatement We are given an array of barcodes. We need to rearrange the barcodes so that no two adjacent barcodes are equal. It is guaranteed that a valid rearrangement always exists. The official constraints state that 1 <= barcodes.length <= 10000 and 1 <= barcodes[i] <= 10000 . Input and Output Item Meaning Input Array barcodes Output Rearranged array with no two adjacent equal elements Function shape: def rearrangeBarcodes(barcodes:...

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LeetCode 1010: Pairs of Songs With Total Durations Divisible by 60

Problem Restatement We are given a list of song durations time . We want to count pairs of songs (i, j) where i < j and: (time[i] + time[j]) % 60 == 0 We need to return the number of such pairs. The official constraints state that 1 <= time.length <= 6 * 10^4 and 1 <= time[i] <= 500 . Input and Output Item Meaning Input Array time of...

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LeetCode 1002: Find Common Characters

Problem Restatement We are given an array of strings words . We need to return a list of all characters that appear in every string in words , including duplicates. If a character appears k times in every string, we include it k times in the answer. The answer can be returned in any order. The official constraints state that 1 <= words.length <= 100 , 1 <= words[i].length <=...

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LeetCode 1072: Flip Columns For Maximum Number of Equal Rows

Problem Restatement We are given an m x n binary matrix. In one operation, we choose any column and flip all values in it (0 → 1, 1 → 0). We want to maximize the number of rows that have all the same value (all-0 or all-1) after any number of column flips. The official constraints state that 1 <= m, n <= 300 . Input and Output Item Meaning...

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LeetCode 1001: Grid Illumination

Problem Restatement We have an n x n grid. We are given an array lamps , where each element is [row_i, col_i] , meaning a lamp is placed at that cell. A lamp illuminates its entire row, its entire column, its main diagonal, and its anti-diagonal. We are also given an array queries , where each query is [row_j, col_j] . For each query: If the queried cell is illuminated,...

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LeetCode 1048: Longest String Chain

Problem Restatement We are given an array of words. A word wordA is a predecessor of wordB if we can insert exactly one letter anywhere in wordA to make it equal to wordB . A word chain is a sequence of words [word_1, word_2, ..., word_k] where each word_i is a predecessor of word_{i+1} . Return the length of the longest word chain. The official constraints state that 1 <=...

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LeetCode 1036: Escape a Large Maze

Problem Restatement We have a 10^6 x 10^6 grid. Some cells are blocked. We are given: blocked : list of blocked cells. source : starting cell. target : target cell. We need to return true if source can reach target , or false if it is impossible. The official constraints state that 0 <= blocked.length <= 200 , and cells are given as [r, c] . Input and Output Item...

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LeetCode 1074: Number of Submatrices That Sum to Target

Problem Restatement We are given an m x n matrix and an integer target . Return the number of non-empty submatrices that sum to target . The official constraints state that 1 <= m, n <= 100 and -1000 <= matrix[i][j] <= 1000 . Input and Output Item Meaning Input Matrix and integer target Output Number of submatrices summing to target Function shape: def numSubmatrixSumTarget(matrix: list[list[int]], target: int) -> int:...

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LeetCode 1086: High Five

Problem Restatement We are given a list of [id, score] pairs. For each student, compute the average of their top 5 scores (integer division). Return the result sorted by student ID. The official constraints state that each student has at least 5 scores. Input and Output Item Meaning Input List of [id, score] pairs Output [[id, top5avg], ...] sorted by id Function shape: def highFive(items: list[list[int]]) -> list[list[int]]: ... Examples...

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LeetCode 1090: Largest Values From Labels

Problem Restatement We have n items, each with a value and a label . We want to choose a subset of items to maximize total value, subject to: At most numWanted items can be chosen. At most useLimit items with the same label can be chosen. The official constraints state that n == values.length == labels.length and 1 <= numWanted, useLimit <= n <= 2 * 10^4 . Input and...

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LeetCode 1015: Smallest Integer Divisible by K

Problem Restatement We need to find the smallest positive integer N consisting only of the digit 1 (a repunit) such that N is divisible by k . In other words, find the smallest n such that: N = 111...1 (n ones) N % k == 0 Return n , or -1 if no such positive integer exists. The official constraints state that 1 <= k <= 10^5 . Input and...

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LeetCode 974: Subarray Sums Divisible by K

Problem Restatement We are given an integer array nums and an integer k . Return the number of non-empty contiguous subarrays whose sum is divisible by k . A subarray is a continuous part of the array. The official constraints are: Constraint Value nums.length 1 <= nums.length <= 3 * 10^4 nums[i] -10^4 <= nums[i] <= 10^4 k 2 <= k <= 10^4 Source: LeetCode problem statement. Input and Output...

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LeetCode 846: Hand of Straights

Problem Restatement We are given an integer array hand , where each value is written on one card. We are also given an integer groupSize . We need to decide whether all cards can be rearranged into groups such that: Rule Meaning Group size Every group has exactly groupSize cards Consecutive values Cards in each group are consecutive Use all cards Every card must be used exactly once Return True...

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LeetCode 923: 3Sum With Multiplicity

Problem Restatement We are given an integer array arr and an integer target . Return the number of index triples: i < j < k such that: arr[i] + arr[j] + arr[k] == target The array can contain duplicate values, and each valid index triple counts separately. Because the answer can be large, return it modulo: 10**9 + 7 The official problem asks for exactly this count of tuples i,...

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LeetCode 970: Powerful Integers

Problem Restatement We are given three integers: x y bound An integer is powerful if it can be written as: x ** i + y ** j where: i >= 0 j >= 0 Return all powerful integers less than or equal to bound . Each value should appear at most once, and the answer may be returned in any order. The constraints are 1 <= x, y <= 100...

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LeetCode 873: Length of Longest Fibonacci Subsequence

Problem Restatement We are given a strictly increasing array arr of positive integers. We need to return the length of the longest Fibonacci-like subsequence. A sequence is Fibonacci-like if: Its length is at least 3 . For every valid position: x[i] + x[i + 1] == x[i + 2] A subsequence is formed by deleting zero or more elements without changing the order of the remaining elements. If no Fibonacci-like...

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LeetCode 966: Vowel Spellchecker

Problem Restatement We are given: wordlist queries For each query, return the corrected word according to these priority rules: If the query exactly matches a word in wordlist , return the query itself. Otherwise, if the query matches a word ignoring capitalization, return the first such word from wordlist . Otherwise, if the query matches a word after treating all vowels as interchangeable, return the first such word from wordlist...

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LeetCode 895: Maximum Frequency Stack

Problem Restatement Design a data structure called FreqStack . It must support two operations: Operation Meaning push(val) Push integer val onto the stack pop() Remove and return the value with highest frequency If multiple values have the same highest frequency, pop() must remove and return the one that was pushed most recently. So this is like a stack, but priority is based on: Higher frequency first. More recent push first...

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LeetCode 916: Word Subsets

Problem Restatement We are given two string arrays: words1 words2 A string b is a subset of string a if every letter in b appears in a with at least the same frequency. For example: "wrr" is a subset of: "warrior" because "warrior" has at least one "w" and at least two "r" characters. But "wrr" is not a subset of: "world" because "world" has only one "r" . A...

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LeetCode 963: Minimum Area Rectangle II

Problem Restatement We are given a list of unique points in the x-y plane. Each point is: [x, y] We need to find the minimum area of any rectangle formed by four of these points. The rectangle sides do not need to be parallel to the axes. A rotated rectangle is allowed. If no rectangle can be formed, return 0 . Answers within 10^-5 of the actual answer are accepted....

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LeetCode 869: Reordered Power of 2

Problem Restatement We are given a positive integer n . We may reorder its digits in any way. The reordered number: Cannot have leading zeroes. Must use exactly the same digits. We need to return True if some reordering forms a power of two. Otherwise, return False . Input and Output Item Meaning Input Positive integer n Output True if some digit reordering forms a power of two Constraint No...

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LeetCode 992: Subarrays with K Different Integers

Problem Restatement We are given an integer array nums and an integer k . A good subarray is a contiguous subarray that contains exactly k different integers. We need to return the number of good subarrays. For example: nums = [1, 2, 1, 2, 3] k = 2 The good subarrays are: [1, 2] [2, 1] [1, 2] [2, 3] [1, 2, 1] [2, 1, 2] [1, 2, 1, 2]...

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LeetCode 893: Groups of Special-Equivalent Strings

Problem Restatement We are given an array words . All strings have the same length. In one move, we can swap two characters only if their indices have the same parity: Swap type Allowed? Even index with even index Yes Odd index with odd index Yes Even index with odd index No Two strings are special-equivalent if one can be changed into the other using any number of these moves....

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LeetCode 750: Number Of Corner Rectangles

Problem Restatement We are given an m x n binary matrix grid . Each cell is either 0 or 1 . We need to count how many axis-aligned rectangles can be formed where all four corner cells are 1 . Only the corners need to be 1 . The cells inside the rectangle and along its edges may be either 0 or 1 . All four corner cells must be...

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LeetCode 771: Jewels and Stones

Problem Restatement We are given two strings: jewels stones Each character in jewels represents a type of stone that is a jewel. Each character in stones represents one stone we have. We need to count how many stones are also jewels. Letters are case-sensitive, so "a" and "A" are different stone types. The official statement also says all characters in jewels are unique. Input and Output Item Meaning Input jewels...

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LeetCode 721: Accounts Merge

Problem Restatement We are given a list of accounts. Each account has this format: [name, email1, email2, ...] Two accounts belong to the same person if they share at least one email address. After merging accounts, each returned account should contain: The person's name. All unique emails belonging to that person, sorted lexicographically. The returned accounts can be in any order. Accounts with the same name do not automatically belong...

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LeetCode 748: Shortest Completing Word

Problem Restatement We are given a string licensePlate and a list of words. A word is called a completing word if it contains every letter from licensePlate . When reading licensePlate : Ignore digits. Ignore spaces. Treat uppercase and lowercase letters as the same. If a letter appears multiple times, the word must contain it at least that many times. We need to return the shortest completing word from words...

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LeetCode 770: Basic Calculator IV

Problem Restatement We are given an algebraic expression as a string. The expression may contain: integers variables + - * parentheses We are also given some variable values through: evalvars evalints For example: evalvars = ["e"] evalints = [1] means: e = 1 We need to simplify the expression after substituting known variables. The result should be returned as a list of string terms. The official format requires variable factors...

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LeetCode 720: Longest Word in Dictionary

Problem Restatement We are given an array of strings: words We need to find the longest word that can be built one character at a time by other words in the array. A word is valid if every prefix formed by removing the last character also exists in the array. For example: "world" is valid only if all these words exist: "w" "wo" "wor" "worl" "world" If there are multiple...

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LeetCode 49: Group Anagrams

Problem Restatement We are given an array of strings strs . We need to group strings that are anagrams of each other. Two strings are anagrams if they contain the same characters with the same frequencies, but possibly in a different order. The answer can be returned in any order. LeetCode states the input strings contain lowercase English letters, with 1 <= strs.length <= 10^4 and 0 <= strs[i].length <=...

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LeetCode 890: Find and Replace Pattern

Problem Restatement We are given a list of strings words and a string pattern . We need to return all words that match the pattern. A word matches the pattern if there is a bijection between letters in pattern and letters in the word. That means: Each pattern character maps to exactly one word character. No two pattern characters map to the same word character. The mapping must be consistent...

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LeetCode 745: Prefix and Suffix Search

Problem Restatement We are given a list of words. We need to design a class: WordFilter that supports queries: f(prefix, suffix) The query should return the largest index of a word that: Starts with prefix Ends with suffix If no such word exists, return: -1 The official problem asks for the maximum index satisfying both prefix and suffix constraints. Input and Output Item Meaning Constructor WordFilter(words) Query f(pref, suff) Output...

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LeetCode 767: Reorganize String

Problem Restatement We are given a string s . We need to rearrange its characters so that no two adjacent characters are the same. Return any valid rearrangement. If no valid rearrangement exists, return an empty string. The problem allows any correct answer, not one specific output. The official statement says to rearrange the characters so adjacent characters are not equal, or return "" if impossible. Input and Output Item...

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LeetCode 41: First Missing Positive

Problem Restatement We are given an unsorted integer array nums . We need to return the smallest positive integer that does not appear in the array. The solution must run in O(n) time and use O(1) auxiliary space. Examples: nums = [1, 2, 0] The positive integers 1 and 2 are present, so the smallest missing positive integer is 3 . nums = [3, 4, -1, 1] The number 1...

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LeetCode 889: Construct Binary Tree from Preorder and Postorder Traversal

Problem Restatement We are given two arrays: Array Meaning preorder Preorder traversal of a binary tree postorder Postorder traversal of the same binary tree The binary tree contains distinct values. We need to reconstruct and return one possible binary tree. If multiple valid trees exist, returning any one of them is accepted. The problem guarantees that both traversals come from the same binary tree. Preorder traversal visits nodes in this...

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LeetCode 765: Couples Holding Hands

Problem Restatement We are given a row of seats. There are n couples, so there are 2n people. People are numbered from 0 to 2n - 1 . Couples are paired in order: Couple People 0 0, 1 1 2, 3 2 4, 5 In general, person x has partner: x ^ 1 This works because: 0 ^ 1 = 1 1 ^ 1 = 0 2 ^ 1 =...

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LeetCode 740: Delete and Earn

Problem Restatement We are given an integer array nums . We may repeatedly choose a number x . When we choose x : We earn x points. Every occurrence of x - 1 is deleted. Every occurrence of x + 1 is deleted. We can continue until no numbers remain. We need to return the maximum total points we can earn. The official problem states that after taking a value...

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LeetCode 914: X of a Kind in a Deck of Cards

Problem Restatement We are given an integer array deck . Each value represents the number written on one card. We need to decide whether we can split the entire deck into groups such that: Rule Meaning Same group size Every group has exactly x cards Minimum group size x > 1 Same card value All cards in one group have the same integer Return True if such a partition is...

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LeetCode 961: N-Repeated Element in Size 2N Array

Problem Restatement We are given an integer array nums . The array has these properties: Property Meaning nums.length == 2 * n The array has even length nums contains n + 1 unique values There are fewer unique values than total values One value appears exactly n times This is the value we must return All other values appear once No other value is duplicated Return the value that appears...

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LeetCode 888: Fair Candy Swap

Problem Restatement Alice and Bob each have several candy boxes. aliceSizes[i] is the number of candies in Alice's i th box. bobSizes[j] is the number of candies in Bob's j th box. They want to exchange exactly one box each so that after the swap, they have the same total number of candies. Return: [alice_box, bob_box] where alice_box is the size of the box Alice gives to Bob, and bob_box...

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LeetCode 763: Partition Labels

Problem Restatement We are given a string s . We need to split it into as many parts as possible so that each letter appears in at most one part. After concatenating all parts in order, we must get the original string back. Return the sizes of the parts. Input and Output Item Meaning Input A string s Output A list of partition sizes Goal Make as many partitions as...

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LeetCode 737: Sentence Similarity II

Problem Restatement We are given two sentences represented as arrays of words: sentence1 sentence2 We are also given a list of similar word pairs: similarPairs Each pair: [x, y] means word x is similar to word y . Two sentences are similar if: They have the same number of words. For every index i , sentence1[i] is similar to sentence2[i] . A word is always similar to itself. Similarity is...

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LeetCode 911: Online Election

Problem Restatement We need to implement a class called TopVotedCandidate . We are given two arrays: persons times The i -th vote was cast for persons[i] at time times[i] . For a query time t , we need to return the candidate who was leading at time t . Votes cast exactly at time t count. If there is a tie, the candidate who received the most recent vote among...

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LeetCode 648: Replace Words

Problem Restatement We are given a dictionary of root words and a sentence . A root can form a longer derivative word. For example: root = "cat" derivative = "cattle" Since "cattle" starts with "cat" , we can replace "cattle" with "cat" . For every word in the sentence, if it starts with one or more roots from the dictionary, replace it with the shortest matching root. If no root...

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LeetCode 710: Random Pick with Blacklist

Problem Restatement We need to design a class that randomly returns an integer from the range: [0, n - 1] Some values are blacklisted and cannot be returned. Every allowed value must have the same probability. The class supports: Operation Meaning Solution(n, blacklist) Initialize the picker pick() Return a uniformly random allowed value The challenge is to make pick() efficient. Input and Output Item Meaning Input Integer n Input Array...

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LeetCode 736: Parse Lisp Expression

Problem Restatement We are given a string expression that represents a Lisp-like expression. We need to evaluate it and return its integer value. An expression can be one of these forms: Form Meaning Integer A positive or negative integer Variable A name whose value comes from a surrounding let add Adds two expressions mult Multiplies two expressions let Assigns variables, then evaluates a final expression The official syntax includes integer...

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LeetCode 645: Set Mismatch

Problem Restatement We are given an array nums of length n . Originally, the array should contain every number from 1 to n exactly once. But an error happened: Error Meaning One number appears twice This is the duplicate number One number is missing This is the missing number We need to return: [duplicate, missing] in that exact order. The official examples include nums = [1, 2, 2, 4] ,...

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LeetCode 760: Find Anagram Mappings

Problem Restatement We are given two integer arrays: nums1 nums2 nums2 is an anagram of nums1 . That means nums2 contains exactly the same values as nums1 , but possibly in a different order. We need to return an array mapping where: mapping[i] = j means: nums1[i] == nums2[j] If there are multiple valid answers, we can return any of them. Input and Output Item Meaning Input Two integer arrays...

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LeetCode 884: Uncommon Words from Two Sentences

Problem Restatement We are given two sentences, s1 and s2 . A word is uncommon if it satisfies both conditions: It appears exactly once in one sentence. It does not appear in the other sentence. Return all uncommon words. The answer may be returned in any order. LeetCode defines each sentence as lowercase words separated by single spaces. Input and Output Item Meaning Input Two strings s1 and s2 Output...

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LeetCode 957: Prison Cells After N Days

Problem Restatement We are given an array cells of length 8 . Each cell is either: 0 # vacant 1 # occupied Every day, the cells change using this rule: If a cell has two adjacent neighbors that are both occupied or both vacant, the cell becomes occupied. Otherwise, it becomes vacant. The first and last cells do not have two neighbors, so they always become vacant after one day....

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LeetCode 859: Buddy Strings

Problem Restatement We are given two strings: s goal We need to return True if we can swap exactly two letters in s so that the result becomes equal to goal . A swap means choosing two different indices i and j , then exchanging: s[i] s[j] If no such swap exists, return False . Input and Output Item Meaning Input s , the original string Input goal , the...

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LeetCode 734: Sentence Similarity

Problem Restatement We are given two sentences represented as arrays of words: sentence1 sentence2 We are also given a list of similar word pairs: similarPairs Each pair: [x, y] means word x is similar to word y . Two sentences are similar if: They have the same number of words. Every word at index i in sentence1 is similar to the word at index i in sentence2 . A word...

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LeetCode 982: Triples with Bitwise AND Equal To Zero

Problem Restatement We are given an integer array nums . We need to count the number of ordered triples of indices (i, j, k) such that: nums[i] & nums[j] & nums[k] == 0 The indices may repeat. For example, (0, 0, 1) is allowed. The triple is ordered, so (0, 1, 2) and (1, 0, 2) are counted as different triples if both satisfy the condition. The official constraints are:...

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LeetCode 616: Add Bold Tag in String

Problem Restatement We are given a string s and an array of strings words . We need to wrap every substring of s that appears in words with a pair of bold tags: <b>...</b> If two matching substrings overlap, they should be wrapped together with one pair of tags. If two matching substrings are adjacent, they should also be combined into one bold region. Return the final string after inserting...

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LeetCode 706: Design HashMap

Problem Restatement We need to design a class called MyHashMap . The class should support three operations: Operation Meaning put(key, value) Insert or update the value for key get(key) Return the value for key , or -1 if missing remove(key) Remove key and its value if it exists We cannot use built-in hash table libraries. A hash map stores key-value pairs. Each key maps to one value. If put is...

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LeetCode 981: Time Based Key-Value Store

Problem Restatement We need to design a data structure called TimeMap . It supports two operations: Operation Meaning set(key, value, timestamp) Store value for key at timestamp get(key, timestamp) Return the value for key with the largest stored timestamp <= timestamp If no stored timestamp is less than or equal to the query timestamp, return an empty string. The important guarantee is that, for each key, calls to set happen...

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LeetCode 705: Design HashSet

Problem Restatement We need to design a class called MyHashSet . The class should support three operations: Operation Meaning add(key) Insert key into the hash set remove(key) Remove key from the hash set if it exists contains(key) Return whether key exists in the hash set We cannot use built-in hash table libraries. The value range is bounded: 0 <= key <= 1_000_000 At most 10,000 calls will be made to...

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LeetCode 756: Pyramid Transition Matrix

Problem Restatement We are given a bottom row of blocks. Each block has a color represented by one letter. We are also given a list of allowed triples. A triple has length 3 . For example: "ABC" means: A B C If two adjacent blocks are A and B , then we may place C on top of them. Starting from the given bottom row, we need to decide whether...

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LeetCode 954: Array of Doubled Pairs

Problem Restatement We are given an integer array arr . We want to reorder the array so that for every pair: (arr[2 * i], arr[2 * i + 1]) the second number equals double the first number: arr[2 * i + 1] = 2 * arr[2 * i] Return true if such a reordering is possible. Otherwise, return false . The official constraints are: Constraint Value 2 <= arr.length <=...

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LeetCode 904: Fruit Into Baskets

Problem Restatement We are given an integer array fruits . Each value represents the fruit type of one tree in a row. We have two baskets. Each basket can hold only one fruit type, but it can hold unlimited fruits of that type. Starting from any tree, we must move right and pick exactly one fruit from every tree. We cannot skip trees. We stop when we reach a tree...

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LeetCode 953: Verifying an Alien Dictionary

Problem Restatement We are given: A list of words called words A string called order The string order describes the alphabet order used in an alien language. We need to check whether the words are sorted lexicographically according to this alien alphabet. Return true if the list is correctly sorted. Otherwise, return false . The official constraints are: Constraint Value 1 <= words.length <= 100 Number of words 1 <=...

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LeetCode 609: Find Duplicate File in System

Problem Restatement We are given a list of directory information strings. Each string has this format: "directory_path file1.txt(content1) file2.txt(content2) ..." For example: "root/a 1.txt(abcd) 2.txt(efgh)" means directory root/a contains: File Content 1.txt abcd 2.txt efgh We need to return groups of duplicate files. Two files are duplicates if they have exactly the same content, even if they have different names or live in different directories. Each returned group must contain...

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LeetCode 535: Encode and Decode TinyURL

Problem Restatement We need to design a class that can shorten a long URL and later recover the original URL. The class has two methods: encode(longUrl) and: decode(shortUrl) encode receives a valid long URL and returns a tiny URL. decode receives a tiny URL generated by the same object and returns the original long URL. The problem does not restrict the algorithm. It only requires that a URL encoded by...

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LeetCode 533: Lonely Pixel II

Problem Restatement We are given an m x n picture. Each cell is either: Character Meaning "B" Black pixel "W" White pixel We are also given an integer target . We need to count black pixels located at some position (r, c) such that both rules are true: Row r contains exactly target black pixels, and column c also contains exactly target black pixels. Every row that has a black...

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LeetCode 532: K-diff Pairs in an Array

Problem Restatement We are given an integer array nums and an integer k . We need to count the number of unique pairs (a, b) such that: abs(a - b) == k The pair must use two different indices in the array. The word unique is important. If the same value pair appears many times because of duplicates, we count it only once. The problem guarantees: 0 <= k So...

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LeetCode 425: Word Squares

Problem Restatement We are given an array of unique strings: words All words have the same length. Return all word squares that can be built from these words. The same word may be used multiple times. A word square is a list of words where the k th row and the k th column read the same string. The answer may be returned in any order. The source statement says...

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LeetCode 424: Longest Repeating Character Replacement

Problem Restatement We are given a string s and an integer k . The string contains only uppercase English letters. In one operation, we may choose any character and change it to any other uppercase English letter. We may perform at most k operations. Return the length of the longest substring that can be made of one repeated letter after at most k replacements. The official examples include s =...

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LeetCode 423: Reconstruct Original Digits from English

Problem Restatement We are given a string s . The string contains shuffled letters from English digit words: "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" The letters are out of order. We must reconstruct the original digits and return them in ascending order. The input is guaranteed to be valid. The constraints allow s.length up to 10^5 . The source examples include "owoztneoer" -> "012" and "fviefuro"...

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LeetCode 500: Keyboard Row

Problem Restatement We are given an array of strings words . Return the words that can be typed using letters from only one row of an American keyboard. The keyboard rows are: Row Letters First row qwertyuiop Second row asdfghjkl Third row zxcvbnm Letter case does not matter. For example, A and a belong to the same row. The word must use letters from only one row. The original word...

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LeetCode 398: Random Pick Index

Problem Restatement We are given an integer array nums . The array may contain duplicate values. We need to implement a class with one operation: pick(target) This operation returns a random index i such that: nums[i] == target If the target appears at multiple indices, each valid index must have equal probability of being returned. The problem guarantees that target exists in nums . The constraints allow nums.length <= 2...

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LeetCode 325: Maximum Size Subarray Sum Equals k

Problem Restatement We are given an integer array nums and an integer k . Return the maximum length of a contiguous subarray whose sum equals k . If no such subarray exists, return 0 . A subarray must be contiguous. The official examples include nums = [1, -1, 5, -2, 3] , k = 3 , whose answer is 4 , because [1, -1, 5, -2] sums to 3 and...

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LeetCode 447: Number of Boomerangs

Problem Restatement We are given a list of distinct points on a 2D plane. Each point is represented as: [x, y] A boomerang is an ordered tuple of points: (i, j, k) such that the distance from i to j equals the distance from i to k . The order matters. So: (i, j, k) and: (i, k, j) are counted as two different boomerangs. The official problem states that...

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LeetCode 350: Intersection of Two Arrays II

Problem Restatement We are given two integer arrays, nums1 and nums2 . Return their intersection. Unlike LeetCode 349, duplicates matter here. Each element in the result must appear as many times as it appears in both arrays. More precisely, if a value appears a times in nums1 and b times in nums2 , it should appear: min(a, b) times in the answer. The result may be returned in any order....

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LeetCode 496: Next Greater Element I

Problem Restatement We are given two arrays: Array Meaning nums1 Query numbers nums2 A larger reference array Every value in both arrays is unique, and every value in nums1 appears in nums2 . For each value x in nums1 , we need to find the first greater number appearing to the right of x inside nums2 . If no greater value exists, return -1 for that number. Input and Output...

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LeetCode 446: Arithmetic Slices II - Subsequence

Problem Restatement We are given an integer array nums . We need to count how many arithmetic subsequences exist in the array. A sequence is arithmetic if: It has at least three elements. The difference between every two consecutive elements is the same. For example: [1, 3, 5, 7, 9] is arithmetic because every step increases by 2 . This is also arithmetic: [7, 7, 7, 7] because every step...

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LeetCode 395: Longest Substring with At Least K Repeating Characters

Problem Restatement We are given a string s and an integer k . We need to return the length of the longest substring where every character in that substring appears at least k times. A substring must be contiguous. If no valid substring exists, return 0 . The string contains only lowercase English letters. The length of s is at most 10^4 , and k can be larger than the...

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LeetCode 349: Intersection of Two Arrays

Problem Restatement We are given two integer arrays, nums1 and nums2 . Return an array containing the intersection of the two arrays. The intersection means the values that appear in both arrays. Each value in the result must be unique, and the result can be returned in any order. The constraints allow values from 0 to 1000 , but a hash set solution works for general integers too. Input and...

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LeetCode 299: Bulls and Cows

Problem Restatement We are given two equal-length digit strings: secret guess We need to return a hint in this format: "xAyB" where: Symbol Meaning x Number of bulls y Number of cows A bull is a digit that has the same value and the same position in both strings. A cow is a digit that exists in both strings but is in the wrong position. Duplicate digits must be counted...

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LeetCode 348: Design Tic-Tac-Toe

Problem Restatement We need to design a Tic-Tac-Toe game played by two players on an n x n board. The class supports two operations: Method Meaning TicTacToe(n) Initializes an n x n board move(row, col, player) Places player at position (row, col) and returns the winner The rules are: Rule Meaning Valid move Every move is guaranteed to be placed on an empty cell Game state No moves are made...

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LeetCode 347: Top K Frequent Elements

Problem Restatement We are given an integer array nums and an integer k . Return the k most frequent elements in the array. The answer may be returned in any order. The problem guarantees that the answer is unique. The constraints allow nums.length up to 10^5 , so the solution must be better than O(n^2) . Input and Output Item Meaning Input Integer array nums and integer k Output The...

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LeetCode 491: Non-decreasing Subsequences

Problem Restatement We are given an integer array nums . We need to return all different non-decreasing subsequences with length at least 2 . A subsequence keeps the original order, but may skip elements. A sequence is non-decreasing if every next value is greater than or equal to the previous value. The answer can be returned in any order. The constraints are 1 <= nums.length <= 15 and -100 <=...

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LeetCode 391: Perfect Rectangle

Problem Restatement We are given a list of axis-aligned rectangles. Each rectangle is represented as: [xi, yi, ai, bi] where: (xi, yi) is the bottom-left corner (ai, bi) is the top-right corner We need to return True if all small rectangles together form one exact rectangular region. That means: There must be no gaps. There must be no overlaps. The final union must be one rectangle. Input and Output Item...

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LeetCode 291: Word Pattern II

Problem Restatement We are given: pattern s We need to check whether s follows pattern . Each character in pattern must map to a non-empty substring of s . The mapping must be bijective: Rule Meaning One character maps to one substring The same pattern character must always use the same substring One substring maps to one character Two different pattern characters cannot use the same substring Full match All...

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LeetCode 290: Word Pattern

Problem Restatement We are given two strings: pattern s pattern contains letters. s contains words separated by spaces. We need to check whether s follows the same pattern. “Follows the same pattern” means there must be a bijection between pattern letters and words. In other words, each pattern letter maps to exactly one word, and each word maps back to exactly one pattern letter. For example: pattern = "abba" s...

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LeetCode 242: Valid Anagram

Problem Restatement We are given two strings: s and t We need to determine whether t is an anagram of s . Two strings are anagrams if: They contain exactly the same characters Every character appears the same number of times The order does not matter. LeetCode examples include: Input: s = "anagram", t = "nagaram" Output: true and: Input: s = "rat", t = "car" Output: false The problem...

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LeetCode 288: Unique Word Abbreviation

Problem Restatement We are given a dictionary of words. We need to design a class that can answer this query: isUnique(word) It returns whether word has a unique abbreviation in the dictionary. The abbreviation rule is: If the word length is less than 3 , the abbreviation is the word itself. Otherwise: first letter + number of middle letters + last letter For example: "dog" -> "d1g" "internationalization" -> "i18n"...

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LeetCode 267: Palindrome Permutation II

Problem Restatement We are given a string s . Return all palindromic permutations of s without duplicates. A palindrome reads the same forward and backward. If no palindromic permutation exists, return an empty list. The answer may be returned in any order. The constraints are 1 <= s.length <= 16 , and s contains only lowercase English letters. Input and Output Item Meaning Input A lowercase string s Output List...

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LeetCode 266: Palindrome Permutation

Problem Restatement We are given a string s . Return True if some permutation of s can form a palindrome. Return False otherwise. A palindrome reads the same forward and backward. For example: "aba" "racecar" "abba" The order of characters in s can be rearranged. We only need to know whether at least one palindrome arrangement exists. The constraints are 1 <= s.length <= 5000 , and s contains only...

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LeetCode 170: Two Sum III - Data Structure Design

Problem Restatement We need to design a data structure that supports two operations: add(number) find(value) The operations behave like this: Operation Meaning add(number) Add the number into the internal data structure find(value) Return True if any pair of numbers sums to value The same number may be added multiple times. The pair must use two different occurrences unless the number appears at least twice. For example: add(1) add(3) add(5) Then:...

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LeetCode 169: Majority Element

Problem Restatement We are given an integer array nums of size n . We need to return the majority element. The majority element is the value that appears more than: n // 2 times. The problem guarantees that a majority element always exists. LeetCode also asks whether we can solve it in linear time and constant extra space. Input and Output Item Meaning Input Integer array nums Output The majority...

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LeetCode 166: Fraction to Recurring Decimal

Problem Restatement We are given two integers: numerator denominator They represent the fraction: numerator / denominator We need to return the decimal representation as a string. If the fractional part repeats forever, we put the repeating part inside parentheses. For example: 1 / 2 = "0.5" 2 / 1 = "2" 2 / 3 = "0.(6)" 4 / 333 = "0.(012)" The denominator is never zero, and the result string...

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LeetCode 314: Binary Tree Vertical Order Traversal

Problem Restatement We are given the root of a binary tree. Return the vertical order traversal of the tree's node values. Vertical order means: Group nodes by column. Output columns from left to right. Inside each column, output nodes from top to bottom. If two nodes have the same row and column, output the left node before the right node. The root starts at column 0 . For any node:...

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LeetCode 389: Find the Difference

Problem Restatement We are given two strings s and t . String t is created by shuffling s and adding one extra letter at a random position. We need to return the added letter. For example: s = "abcd" t = "abcde" The added letter is: "e" The strings contain only lowercase English letters. Also, t always has exactly one more character than s . Input and Output Item Meaning...

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LeetCode 388: Longest Absolute File Path

Problem Restatement We are given a string input that represents a file system. Each file or directory appears on its own line. A newline character means a new entry: "\n" A tab character at the start of a line tells us the depth: "\t" A file is identified by having a dot . in its name. We need to return the length of the longest absolute path to any file....

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LeetCode 438: Find All Anagrams in a String

Problem Restatement We are given two strings: Name Meaning s The string we search inside p The pattern string We need to return all starting indices in s where an anagram of p begins. An anagram uses the same characters with the same frequencies, but the order can be different. For example: "abc", "bac", "cba" are all anagrams of each other. The official problem asks us to return all start...

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LeetCode 37: Sudoku Solver

Problem Restatement We are given a partially filled 9 x 9 Sudoku board. Empty cells are represented by: "." We must fill the empty cells so that the final board satisfies Sudoku rules: Rule Meaning Row rule Each digit 1 to 9 appears exactly once in every row Column rule Each digit 1 to 9 appears exactly once in every column Box rule Each digit 1 to 9 appears exactly...

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LeetCode 387: First Unique Character in a String

Problem Restatement We are given a string s . We need to find the first character that appears exactly once in the string and return its index. If every character appears more than once, return -1 . The string contains only lowercase English letters, and its length can be up to 10^5 . Input and Output Item Meaning Input A string s Output Index of the first non-repeating character Return...

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LeetCode 437: Path Sum III

Problem Restatement We are given the root of a binary tree and an integer targetSum . We need to count how many paths have node values that add up to targetSum . A valid path: Rule Meaning Can start anywhere It does not have to start at the root Can end anywhere It does not have to end at a leaf Must go downward It can only move from parent...

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LeetCode 187: Repeated DNA Sequences

Problem Restatement We are given a string s representing a DNA sequence. A DNA sequence contains only these four characters: Character Meaning A Adenine C Cytosine G Guanine T Thymine We need to find all 10-letter-long substrings that appear more than once in s . The answer can be returned in any order. The official statement asks for all 10-letter-long sequences that occur more than once in a DNA molecule....

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LeetCode 36: Valid Sudoku

Problem Restatement We are given a 9 x 9 Sudoku board. Each cell contains either: "." or a digit from: "1" to "9" The board does not need to be solved. We only need to check whether the filled cells obey Sudoku rules. A board is valid when: Rule Meaning Row rule Each digit 1 to 9 appears at most once in every row Column rule Each digit 1 to...

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LeetCode 409: Longest Palindrome

Problem Restatement We are given a string s . The string contains lowercase and uppercase English letters. We may rearrange the letters in any order. Return the length of the longest palindrome that can be built using those letters. Letters are case-sensitive, so: "A" != "a" The problem asks for the length only, not the palindrome itself. The constraints are 1 <= s.length <= 2000 , and the string contains...

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LeetCode 311: Sparse Matrix Multiplication

Problem Restatement We are given two sparse matrices mat1 and mat2 . A sparse matrix contains many zero values. We need to return the matrix product: mat1 * mat2 If mat1 has size m x k , and mat2 has size k x n , then the result has size m x n . For each result cell: answer[i][j] we compute the dot product of row i from mat1 and...

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LeetCode 340: Longest Substring with At Most K Distinct Characters

Problem Restatement We are given a string s and an integer k . Return the length of the longest substring of s that contains at most k distinct characters. A substring must be contiguous. For example: s = "eceba" k = 2 The substring "ece" contains only two distinct characters: e, c Its length is 3 , so the answer is 3 . Input and Output Item Meaning Input A...

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LeetCode 359: Logger Rate Limiter

Problem Restatement We need to design a logger system. The logger receives messages with timestamps. Each message should be printed only if the same message was not printed in the last 10 seconds. Given: timestamp: int message: str Return: True if the message should be printed. Return: False if the message should be blocked. A message printed at timestamp t prevents the same message from being printed again until timestamp...

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LeetCode 336: Palindrome Pairs

Problem Restatement We are given a 0-indexed array of unique strings words . Return all pairs of different indices (i, j) such that: words[i] + words[j] is a palindrome. A palindrome reads the same forward and backward. For example: "abccba" "racecar" "" are palindromes. The problem asks for all valid pairs, not only one pair. The words are unique, and LeetCode asks for an algorithm close to the total input...

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LeetCode 159: Longest Substring with At Most Two Distinct Characters

Problem Restatement We are given a string s . We need to return the length of the longest substring that contains at most two distinct characters. A substring must be contiguous. For example: s = "eceba" The substring: "ece" contains only two distinct characters: "e", "c" Its length is: 3 So the answer is: 3 LeetCode gives examples such as s = "eceba" with output 3 , and s =...

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LeetCode 383: Ransom Note

Problem Restatement We are given two strings: ransomNote magazine We need to decide whether ransomNote can be built using letters from magazine . Each letter from magazine can be used at most once. So we need enough copies of every character required by ransomNote . For example, if ransomNote = "aa" and magazine = "ab" , the answer is False , because magazine has only one a . Input and...

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LeetCode 432: All O'one Data Structure

Problem Restatement We need to design a data structure that stores string counts. It must support these operations: Operation Meaning inc(key) Increase key count by 1 ; insert it with count 1 if missing dec(key) Decrease key count by 1 ; remove it if count becomes 0 getMaxKey() Return any key with the largest count getMinKey() Return any key with the smallest count If the data structure is empty, both...

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LeetCode 356: Line Reflection

Problem Restatement We are given a list of points on a 2D plane. Each point is represented as: [x, y] We need to decide whether there exists a vertical line such that reflecting every point across that line gives the same set of points. A vertical line has this form: x = c The line is parallel to the y-axis. When a point (x, y) is reflected across x =...

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LeetCode 229: Majority Element II

Problem Restatement We are given an integer array nums of size n . We need to return all elements that appear more than: floor(n / 3) times. The answer can be returned in any order. LeetCode examples include nums = [3,2,3] , which returns [3] , and nums = [1,2] , which returns [1,2] . The constraints are 1 <= nums.length <= 5 * 10^4 and -10^9 <= nums[i] <=...

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LeetCode 30: Substring with Concatenation of All Words

Problem Restatement We are given a string s and an array words . Every word in words has the same length. We need to find every starting index in s where a substring is made by concatenating all words in words exactly once, in any order, with no extra characters between them. Return the starting indices in any order. For example, if: words = ["ab", "cd", "ef"] then valid concatenations...

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LeetCode 381: Insert Delete GetRandom O(1) - Duplicates Allowed

Problem Restatement We need to implement a data structure called RandomizedCollection . It behaves like a multiset, so duplicate values are allowed. It must support three operations in average O(1) time: Operation Meaning insert(val) Insert one occurrence of val remove(val) Remove one occurrence of val if present getRandom() Return a random element from the current collection For getRandom() , every stored occurrence has equal probability. So if the collection contains:...

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LeetCode 355: Design Twitter

Problem Restatement We need to design a simplified Twitter system. The system supports four operations: Method Meaning postTweet(userId, tweetId) User userId posts a tweet with ID tweetId getNewsFeed(userId) Return up to 10 most recent tweet IDs visible to userId follow(followerId, followeeId) followerId starts following followeeId unfollow(followerId, followeeId) followerId stops following followeeId A user's news feed contains: Tweets posted by the user themself. Tweets posted by users they follow. The feed...

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LeetCode 380: Insert Delete GetRandom O(1)

Problem Restatement We need to implement a data structure called RandomizedSet . It supports three operations: Operation Meaning insert(val) Insert val if it does not already exist remove(val) Remove val if it exists getRandom() Return a random element from the current set Each operation must run in average O(1) time. For getRandom() , every element currently in the set must have the same probability of being returned. The problem guarantees...

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LeetCode 480: Sliding Window Median

Problem Restatement We are given an integer array nums and an integer k . A window of size k starts at the left side of the array and moves one step to the right each time. For every window, we need to return the median of the numbers inside that window. The median is defined like this: Window size Median Odd The middle value after sorting Even The average of...

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LeetCode 128: Longest Consecutive Sequence

Problem Restatement We are given an unsorted integer array nums . We need to return the length of the longest consecutive sequence of numbers. The numbers do not need to be adjacent in the array. They only need to exist somewhere in the array. The problem also requires an O(n) time algorithm. Sorting would take O(n log n) , so sorting does not satisfy the required bound. Examples Example 1:...

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LeetCode 13: Roman to Integer

Problem Restatement We are given a Roman numeral string s . We need to convert it into an integer. Roman numerals use seven symbols: Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 Usually, Roman numerals are written from largest to smallest. For example: VIII = 5 + 1 + 1 + 1 = 8 But there are six subtractive cases: Roman Value...

Practice › LeetCode › LeetCode (archived) › LeetCode 00xx ›

LeetCode 12: Integer to Roman

Problem Restatement We are given an integer num . We need to convert it into a Roman numeral string. Roman numerals use seven base symbols: Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 The input satisfies: 1 <= num <= 3999 The LeetCode statement also defines the subtractive forms: Value Roman 4 IV 9 IX 40 XL 90 XC 400 CD 900...

Wiki › Algorithms › 02. Data Structures › 1. Core Data Structures and Operations ›

1.6 Hash Tables and Sets

1.6 Hash tables and sets, 60 index slug name 1 hash-table Hash Table 2 hash-function Hash Function 3 universal-hashing Universal Hashing 4 multiplicative-hashing Multiplicative Hashing 5 division-hashing Division Hashing 6 polynomial-hashing Polynomial Hashing 7 string-hashing String Hashing 8 rolling-hash Rolling Hash 9 cryptographic-hash Cryptographic Hash 10 open-addressing Open Addressing 11 linear-probing Linear Probing 12 quadratic-probing Quadratic Probing 13 double-hashing Double Hashing 14 robin-hood-hashing Robin Hood Hashing 15 hopscotch-hashing Hopscotch Hashing...

Wiki › Algorithms › 01. Searching and Sorting › 3. Hashing and Table Search ›

Hash Table Lookup

Hash Table Lookup Hash table lookup retrieves a value associated with a key by computing a hash and using it to index into a table. It provides constant expected time for search under standard assumptions. You use it when fast membership or key value access is required, especially for repeated queries over large datasets. Problem Given a hash table storing key value pairs and a query key $k$, return the...

#hash-table

LeetCode 3926 - Count Valid Word Occurrences

LeetCode 3843 - First Element with Unique Frequency

LeetCode 3672 - Sum of Weighted Modes in Subarrays

LeetCode 3761 - Minimum Absolute Distance Between Mirror Pairs

LeetCode 3581 - Count Odd Letters from Number

LeetCode 3934 - Smallest Unique Subarray

LeetCode 3927 - Minimize Array Sum Using Divisible Replacements

LeetCode 3923 - Minimum Generations to Target Point

LeetCode 3859 - Count Subarrays With K Distinct Integers

LeetCode 3810 - Minimum Operations to Reach Target Array

LeetCode 3785 - Minimum Swaps to Avoid Forbidden Values

LeetCode 3740 - Minimum Distance Between Three Equal Elements I

LeetCode 3719 - Longest Balanced Subarray I

LeetCode 3713 - Longest Balanced Substring I

LeetCode 3678 - Smallest Absent Positive Greater Than Average

LeetCode 3659 - Partition Array Into K-Distinct Groups

LeetCode 3629 - Minimum Jumps to Reach End via Prime Teleportation

LeetCode 3623 - Count Number of Trapezoids I

LeetCode 3583 - Count Special Triplets

LeetCode 3551 - Minimum Swaps to Sort by Digit Sum

LeetCode 3568 - Minimum Moves to Clean the Classroom

LeetCode 3548 - Equal Sum Grid Partition II

LeetCode 3531 - Count Covered Buildings

LeetCode 3522 - Calculate Score After Performing Instructions

LeetCode 3487 - Maximum Unique Subarray Sum After Deletion

LeetCode 3483 - Unique 3-Digit Even Numbers

LeetCode 3431 - Minimum Unlocked Indices to Sort Nums

LeetCode 3443 - Maximum Manhattan Distance After K Changes

LeetCode 3434 - Maximum Frequency After Subarray Operation

LeetCode 3404 - Count Special Subsequences

LeetCode 3381 - Maximum Subarray Sum With Length Divisible by K

LeetCode 2933 - High-Access Employees

LeetCode 1787 - Make the XOR of All Segments Equal to Zero

LeetCode 3158 - Find the XOR of Numbers Which Appear Twice

LeetCode 3438 - Find Valid Pair of Adjacent Digits in String

LeetCode 3890 - Integers With Multiple Sum of Two Cubes

LeetCode 3889 - Mirror Frequency Distance

LeetCode 3885 - Design Event Manager

LeetCode 3790 - Smallest All-Ones Multiple

LeetCode 3879 - Maximum Distinct Path Sum in a Binary Tree

LeetCode 3784 - Minimum Deletion Cost to Make All Characters Equal

LeetCode 3779 - Minimum Number of Operations to Have Distinct Elements

LeetCode 3873 - Maximum Points Activated with One Addition

LeetCode 3868 - Minimum Cost to Equalize Arrays Using Swaps

LeetCode 3866 - First Unique Even Element

LeetCode 3773 - Maximum Number of Equal Length Runs

LeetCode 3860 - Unique Email Groups

LeetCode 3852 - Smallest Pair With Different Frequencies

LeetCode 3853 - Merge Close Characters

LeetCode 3851 - Maximum Requests Without Violating the Limit

LeetCode 3846 - Total Distance to Type a String Using One Finger

LeetCode 3842 - Toggle Light Bulbs

LeetCode 3839 - Number of Prefix Connected Groups

LeetCode 3837 - Delayed Count of Equal Elements

LeetCode 3829 - Design Ride Sharing System

LeetCode 3749 - Evaluate Valid Expressions

LeetCode 3741 - Minimum Distance Between Three Equal Elements II

LeetCode 3803 - Count Residue Prefixes

LeetCode 3739 - Count Subarrays With Majority Element II

LeetCode 3737 - Count Subarrays With Majority Element I

LeetCode 3731 - Find Missing Elements

LeetCode 3822 - Design Order Management System

LeetCode 3728 - Stable Subarrays With Equal Boundary and Interior Sum

LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters

LeetCode 3815 - Design Auction System

LeetCode 3721 - Longest Balanced Subarray II

LeetCode 3720 - Lexicographically Smallest Permutation Greater Than Target

LeetCode 3714 - Longest Balanced Substring II

LeetCode 3712 - Sum of Elements With Frequency Divisible by K

LeetCode 3811 - Number of Alternating XOR Partitions

LeetCode 3805 - Count Caesar Cipher Pairs

LeetCode 3804 - Number of Centered Subarrays

LeetCode 3692 - Majority Frequency Characters

LeetCode 3795 - Minimum Subarray Length With Distinct Sum At Least K

LeetCode 3755 - Find Maximum Balanced XOR Subarray Length

LeetCode 3729 - Count Distinct Subarrays Divisible by K in Sorted Array

LeetCode 3718 - Smallest Missing Multiple of K

LeetCode 3690 - Split and Merge Array Transformation

LeetCode 3679 - Minimum Discards to Balance Inventory