LeetCode 921: Minimum Add to Make Parentheses Valid
A clear explanation of making a parentheses string valid using greedy counting.
Tag
Counting
37 notes
LeetCode 916: Word Subsets
A clear explanation of finding universal words by merging character frequency requirements from words2.
LeetCode 884: Uncommon Words from Two Sentences
A clear explanation of finding uncommon words by counting word frequencies across both sentences.
LeetCode 621: Task Scheduler
A clear explanation of Task Scheduler using frequency counting and the greedy block formula.
LeetCode 997: Find the Town Judge
A clear explanation of identifying the town judge using trust indegree and outdegree counts.
LeetCode 996: Number of Squareful Arrays
A clear explanation of counting unique permutations where every adjacent pair sums to a perfect square using backtracking.
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 825: Friends Of Appropriate Ages
A counting solution for computing how many directed friend requests are allowed by age rules.
LeetCode 819: Most Common Word
A hash map and string parsing solution for finding the most frequent non-banned word in a paragraph.
LeetCode 811: Subdomain Visit Count
A hash map solution for accumulating visit counts across domains and all of their parent subdomains.
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 697: Degree of an Array
Find the shortest contiguous subarray with the same degree as the whole array using frequency counts and first occurrence indices.
LeetCode 696: Count Binary Substrings
Count substrings with equal consecutive groups of 0s and 1s using run lengths.
LeetCode 242: Valid Anagram
A clear explanation of checking whether two strings are anagrams using character frequency counting.
LeetCode 233: Number of Digit One
A detailed explanation of counting how many times digit one appears from 0 to n using positional digit analysis.
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 594: Longest Harmonious Subsequence
A clear hash map solution for finding the longest subsequence whose maximum and minimum differ by exactly one.
LeetCode 552: Student Attendance Record II
A clear explanation of Student Attendance Record II using dynamic programming over absence count and late streak.
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 531: Lonely Pixel I
A clear explanation of counting black pixels that are alone in both their row and column.
LeetCode 463: Island Perimeter
A clear explanation of counting the perimeter of an island in a grid by adding land-cell edges and subtracting shared edges.
LeetCode 454: 4Sum II
A clear explanation of counting zero-sum tuples across four arrays using pair sums and a hash map.
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 419: Battleships in a Board
A clear explanation of counting battleships in a board using one-pass observation without modifying the grid.
LeetCode 409: Longest Palindrome
A clear explanation of finding the longest palindrome length that can be built from given letters using character counts.
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 299: Bulls and Cows
A counting solution for producing the Bulls and Cows hint while handling duplicate digits correctly.
LeetCode 266: Palindrome Permutation
A clear explanation of the Palindrome Permutation problem using character parity counting.
LeetCode 169: Majority Element
A clear explanation of finding the element that appears more than half the time using Boyer-Moore voting.
5.8 Counting Maps
Track frequency counts for keys using a hash map that increments a counter on each insertion of an existing key.
1.1 Tally Systems and Artifacts
Early counting through marks, objects, and physical recording systems.
Chapter 1. Prehistoric and Early Counting
How early humans developed counting, measurement, and basic mathematical thinking before formal notation.
05. Combinatorics
Counting, arrangement, structure, and extremal behavior of finite and discrete systems.