LeetCode 974: Subarray Sums Divisible by K
A clear explanation of counting subarrays whose sum is divisible by k using prefix sums and remainder frequencies.
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Hash-Table
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LeetCode 970: Powerful Integers
A clear explanation of generating all powerful integers using bounded powers and a set.
LeetCode 966: Vowel Spellchecker
A clear explanation of implementing a spellchecker with exact, case-insensitive, and vowel-error matching.
LeetCode 963: Minimum Area Rectangle II
A clear explanation of finding the minimum-area rectangle from points when the rectangle may be rotated.
LeetCode 961: N-Repeated Element in Size 2N Array
A clear explanation of finding the element repeated N times using a hash set.
LeetCode 957: Prison Cells After N Days
A clear explanation of simulating prison cell transitions efficiently using cycle detection.
LeetCode 954: Array of Doubled Pairs
A clear explanation of checking whether an array can be reordered into pairs where one number is double the other.
LeetCode 953: Verifying an Alien Dictionary
A clear explanation of checking whether words are sorted according to a custom alien alphabet order.
LeetCode 846: Hand of Straights
A clear explanation of the Hand of Straights problem using sorting, frequency counting, and greedy grouping.
LeetCode 923: 3Sum With Multiplicity
A clear explanation of counting index triplets with duplicate values using frequency counts and combinatorics.
LeetCode 916: Word Subsets
A clear explanation of finding universal words by merging character frequency requirements from words2.
LeetCode 914: X of a Kind in a Deck of Cards
A clear explanation of checking whether card counts share a common group size using the greatest common divisor.
LeetCode 911: Online Election
A clear explanation of Online Election using preprocessing and binary search over vote times.
LeetCode 904: Fruit Into Baskets
A clear explanation of Fruit Into Baskets using a sliding window with at most two distinct fruit types.
LeetCode 895: Maximum Frequency Stack
A clear explanation of designing a stack that pops the most frequent value, breaking ties by most recent insertion.
LeetCode 893: Groups of Special-Equivalent Strings
A clear explanation of counting special-equivalent string groups by building canonical signatures from even and odd positions.
LeetCode 890: Find and Replace Pattern
A clear explanation of finding words that match a pattern using bijective character mapping.
LeetCode 889: Construct Binary Tree from Preorder and Postorder Traversal
A clear explanation of reconstructing a binary tree from preorder and postorder traversals using recursion and index ranges.
LeetCode 888: Fair Candy Swap
A clear explanation of finding one candy box swap that makes Alice and Bob have equal total candies.
LeetCode 884: Uncommon Words from Two Sentences
A clear explanation of finding uncommon words by counting word frequencies across both sentences.
LeetCode 992: Subarrays with K Different Integers
A clear explanation of counting subarrays with exactly k distinct integers using the at-most-k sliding window trick.
LeetCode 982: Triples with Bitwise AND Equal To Zero
A clear explanation of counting ordered triples whose bitwise AND is zero using pairwise AND counts.
LeetCode 981: Time Based Key-Value Store
A clear explanation of designing a time-based key-value store using a hash map and binary search.
LeetCode 873: Length of Longest Fibonacci Subsequence
A clear explanation of finding the longest Fibonacci-like subsequence using dynamic programming and value-to-index lookup.
LeetCode 869: Reordered Power of 2
A clear explanation of checking whether the digits of a number can be reordered to form a power of two using digit frequency signatures.
LeetCode 859: Buddy Strings
A clear explanation of checking whether one swap in a string can make it equal to another string.
LeetCode 771: Jewels and Stones
A clear explanation of counting how many stones are jewels using a hash set for fast membership checks.
LeetCode 770: Basic Calculator IV
A clear explanation of simplifying algebraic expressions by parsing, substituting variables, and combining polynomial terms.
LeetCode 767: Reorganize String
A clear explanation of rearranging characters so no two adjacent characters are equal using a greedy max heap.
LeetCode 765: Couples Holding Hands
A clear explanation of minimizing swaps so every couple sits together using greedy position tracking.
LeetCode 763: Partition Labels
A clear explanation of partitioning a string into the maximum number of parts so each character appears in at most one part.
LeetCode 760: Find Anagram Mappings
A clear explanation of mapping each element in one array to a matching index in its anagram using a hash map.
LeetCode 756: Pyramid Transition Matrix
A clear explanation of solving Pyramid Transition Matrix using backtracking and memoization over pyramid rows.
LeetCode 721: Accounts Merge
A clear explanation of merging accounts that share emails using union find and sorted email groups.
LeetCode 720: Longest Word in Dictionary
A clear explanation of finding the longest buildable word using sorting and a hash set.
LeetCode 710: Random Pick with Blacklist
A clear explanation of selecting a uniformly random integer while excluding blacklisted values using remapping and hashing.
LeetCode 706: Design HashMap
A clear explanation of designing a hash map without using built-in hash table libraries.
LeetCode 705: Design HashSet
A clear explanation of designing a hash set without using built-in hash table libraries.
LeetCode 750: Number Of Corner Rectangles
Count axis-aligned rectangles whose four corners are 1 using column-pair frequency counting.
LeetCode 748: Shortest Completing Word
Find the shortest word that contains all required license plate letters using frequency counting.
LeetCode 745: Prefix and Suffix Search
Support fast prefix and suffix queries by indexing every prefix-suffix combination with the largest word index.
LeetCode 740: Delete and Earn
Transform the problem into House Robber dynamic programming by grouping equal values into total points.
LeetCode 737: Sentence Similarity II
Check sentence similarity with transitive word relationships using union-find.
LeetCode 736: Parse Lisp Expression
Evaluate a Lisp-like expression with integers, variables, let bindings, addition, multiplication, and lexical scope.
LeetCode 734: Sentence Similarity
Check whether two word arrays are sentence-similar using a hash set of symmetric similar word pairs.
LeetCode 648: Replace Words
A trie-based solution for replacing each derivative word with the shortest matching root.
LeetCode 645: Set Mismatch
A counting and math solution for finding the duplicated number and the missing number in a corrupted set.
LeetCode 616: Add Bold Tag in String
A string-marking guide for adding bold tags around all matched words while merging overlapping and adjacent bold regions.
LeetCode 609: Find Duplicate File in System
A hash map guide for grouping file paths by identical file content and returning only duplicate groups.
LeetCode 242: Valid Anagram
A clear explanation of checking whether two strings are anagrams using character frequency counting.
LeetCode 229: Majority Element II
A clear explanation of finding all elements that appear more than n/3 times using the extended Boyer-Moore voting algorithm.
LeetCode 49: Group Anagrams
A clear explanation of Group Anagrams using a hash map keyed by each word's sorted character signature.
LeetCode 41: First Missing Positive
A clear explanation of the First Missing Positive problem using in-place index placement to achieve O(n) time and O(1) extra space.
LeetCode 535: Encode and Decode TinyURL
A clear explanation of designing a simple URL encoder and decoder using a hash map and generated keys.
LeetCode 533: Lonely Pixel II
A clear explanation of counting black lonely pixels using row counts, column counts, and duplicate row patterns.
LeetCode 532: K-diff Pairs in an Array
A clear explanation of counting unique pairs whose absolute difference is k using frequency counting.
LeetCode 500: Keyboard Row
A clear explanation of filtering words that can be typed using only one row of an American keyboard.
LeetCode 496: Next Greater Element I
A clear explanation of finding the next greater element using a monotonic decreasing stack and hash map.
LeetCode 491: Non-decreasing Subsequences
A clear explanation of generating all distinct non-decreasing subsequences using DFS, backtracking, and per-level duplicate control.
LeetCode 480: Sliding Window Median
A clear explanation of maintaining the median of each fixed-size window using two heaps and lazy deletion.
LeetCode 447: Number of Boomerangs
Count ordered boomerang tuples by fixing each point as the center and grouping other points by squared distance.
LeetCode 446: Arithmetic Slices II - Subsequence
Count arithmetic subsequences of length at least three using dynamic programming with one hash map per ending index.
LeetCode 438: Find All Anagrams in a String
Find all starting indices where an anagram of p appears in s using a fixed-size sliding window.
LeetCode 437: Path Sum III
Count downward paths in a binary tree whose values sum to targetSum using DFS and prefix sums.
LeetCode 432: All O'one Data Structure
Design a data structure that supports increment, decrement, get minimum key, and get maximum key in average O(1) time.
LeetCode 425: Word Squares
A clear explanation of building all word squares using backtracking with prefix pruning.
LeetCode 424: Longest Repeating Character Replacement
A clear explanation of finding the longest substring that can become all one letter using a sliding window.
LeetCode 423: Reconstruct Original Digits from English
A clear explanation of reconstructing digits from shuffled English words using character frequency counts and unique identifying letters.
LeetCode 409: Longest Palindrome
A clear explanation of finding the longest palindrome length that can be built from given letters using character counts.
LeetCode 398: Random Pick Index
A clear explanation of picking a uniformly random index for a target value using reservoir sampling, with an alternative hash map approach.
LeetCode 395: Longest Substring with At Least K Repeating Characters
A clear explanation of finding the longest substring where every character appears at least k times using divide and conquer.
LeetCode 391: Perfect Rectangle
A clear explanation of checking whether many small axis-aligned rectangles form one exact rectangular cover using area and corner parity.
LeetCode 389: Find the Difference
A clear explanation of finding the extra character added to a shuffled string using counting and XOR.
LeetCode 388: Longest Absolute File Path
A clear explanation of computing the longest absolute path to a file from a serialized file system string using path lengths by depth.
LeetCode 387: First Unique Character in a String
A clear explanation of finding the first non-repeating character in a string using character frequency counting.
LeetCode 383: Ransom Note
A clear explanation of checking whether one string can be constructed from another using character frequency counting.
LeetCode 381: Insert Delete GetRandom O(1) - Duplicates Allowed
A clear explanation of designing a randomized multiset with average O(1) insert, remove, and getRandom operations.
LeetCode 380: Insert Delete GetRandom O(1)
A clear explanation of designing a randomized set with average O(1) insert, remove, and getRandom operations.
LeetCode 359: Logger Rate Limiter
A clear explanation of designing a logger that prints each message at most once every 10 seconds using a hash map.
LeetCode 356: Line Reflection
A clear explanation of checking whether 2D points are symmetric around a vertical line using min and max x-coordinates.
LeetCode 355: Design Twitter
A clear explanation of implementing a simplified Twitter using hash maps, sets, timestamps, and a heap.
LeetCode 350: Intersection of Two Arrays II
A clear explanation of Intersection of Two Arrays II using frequency counting.
LeetCode 349: Intersection of Two Arrays
A clear explanation of Intersection of Two Arrays using hash sets for uniqueness and fast lookup.
LeetCode 348: Design Tic-Tac-Toe
A clear explanation of Design Tic-Tac-Toe using row, column, and diagonal counters for constant-time winner checks.
LeetCode 347: Top K Frequent Elements
A clear explanation of Top K Frequent Elements using frequency counting and bucket sort.
LeetCode 340: Longest Substring with At Most K Distinct Characters
A clear explanation of Longest Substring with At Most K Distinct Characters using a sliding window and character counts.
LeetCode 336: Palindrome Pairs
A clear explanation of Palindrome Pairs using reversed-word lookup and palindrome split checks.
LeetCode 325: Maximum Size Subarray Sum Equals k
A clear explanation of Maximum Size Subarray Sum Equals k using prefix sums and earliest-index hashing.
LeetCode 314: Binary Tree Vertical Order Traversal
A clear explanation of Binary Tree Vertical Order Traversal using BFS with column indices.
LeetCode 311: Sparse Matrix Multiplication
A clear explanation of Sparse Matrix Multiplication using non-zero entries to avoid wasted work.
LeetCode 299: Bulls and Cows
A counting solution for producing the Bulls and Cows hint while handling duplicate digits correctly.
LeetCode 291: Word Pattern II
A backtracking solution for matching a pattern string to a target string using a bijective character-to-substring mapping.
LeetCode 290: Word Pattern
A hash map solution for checking whether a pattern string and a space-separated word string form a bijection.
LeetCode 288: Unique Word Abbreviation
A hash map design for checking whether a word's abbreviation is unique in a dictionary.
LeetCode 267: Palindrome Permutation II
A clear explanation of the Palindrome Permutation II problem using character counts and backtracking over half of the palindrome.
LeetCode 266: Palindrome Permutation
A clear explanation of the Palindrome Permutation problem using character parity counting.
LeetCode 187: Repeated DNA Sequences
A clear explanation of finding repeated 10-letter DNA substrings using a fixed-size sliding window and hash sets.
LeetCode 170: Two Sum III - Data Structure Design
A clear explanation of designing a data structure that supports add and find operations for pair sums.
LeetCode 169: Majority Element
A clear explanation of finding the element that appears more than half the time using Boyer-Moore voting.
LeetCode 166: Fraction to Recurring Decimal
A clear explanation of converting a fraction into decimal form and detecting repeating fractional parts with a hash map.
LeetCode 159: Longest Substring with At Most Two Distinct Characters
A clear explanation of finding the longest substring with at most two distinct characters using a sliding window.
LeetCode 128: Longest Consecutive Sequence
Find the longest run of consecutive integers in an unsorted array using a hash set and sequence-start detection.
LeetCode 37: Sudoku Solver
A clear explanation of solving a Sudoku board using backtracking and constraint checking.
LeetCode 36: Valid Sudoku
A clear explanation of checking whether a partially filled Sudoku board is valid using hash sets.
LeetCode 30: Substring with Concatenation of All Words
A clear explanation of finding all starting indices where a substring is formed by concatenating every word exactly once.
LeetCode 13: Roman to Integer
A detailed explanation of converting a Roman numeral string into an integer using symbol values and the subtraction rule.
LeetCode 12: Integer to Roman
A detailed explanation of converting an integer into a Roman numeral using a fixed value-symbol table and greedy subtraction.
1.6 Hash Tables and Sets
Hash functions, collision resolution strategies, open addressing, chaining, dynamic resizing, perfect hashing, concurrent maps, and probabilistic filters.
Hash Table Lookup
Retrieve a value by computing a hash and probing a table for the corresponding key.