counting-sort

Practice › LeetCode › LeetCode 35xx ›

LeetCode 3517 - Smallest Palindromic Rearrangement I

LeetCode Problem 3517 Difficulty: 🟡 Medium Topics: String, Sorting, Counting Sort Solution Problem Understanding The problem asks us to take a palindromic string s and return its lexicographically smallest palindromic permutation . In other words, we need to rearrange the characters of s into a palindrome that is the smallest possible if you were to compare it alphabetically with all other palindromic arrangements. The input string s is guaranteed to...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3088 - Make String Anti-palindrome

LeetCode Problem 3088 Difficulty: 🔴 Hard Topics: String, Greedy, Sorting, Counting Sort Solution Problem Understanding We are given a string s of even length, and we may swap any two characters any number of times. Since unrestricted swapping allows us to rearrange the string arbitrarily, the real task is to determine whether some permutation of the characters can form an anti-palindrome. A string is an anti-palindrome if every character differs...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3189 - Minimum Moves to Get a Peaceful Board

LeetCode Problem 3189 Difficulty: 🟡 Medium Topics: Array, Greedy, Sorting, Counting Sort Solution Problem Understanding In this problem, we are given the positions of n rooks placed on an n x n chessboard. Each rook is represented as a pair [xi, yi] , where xi is the row index and yi is the column index. A peaceful board is defined as a board where: Every row contains exactly one rook...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 274 - H-Index

LeetCode Problem 274 Difficulty: 🟡 Medium Topics: Array, Sorting, Counting Sort Solution LeetCode 274, H-Index Problem Understanding The problem gives an array called citations , where each element represents how many citations a research paper has received. Each index corresponds to one paper, and the value at that index is the number of times that paper has been cited. We must compute the researcher's h-index . The h-index is defined...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 912 - Sort an Array

LeetCode Problem 912 Difficulty: 🟡 Medium Topics: Array, Divide and Conquer, Sorting, Heap (Priority Queue), Merge Sort, Bucket Sort, Radix Sort, Counting Sort Solution Problem Understanding The problem asks us to sort an integer array in ascending order without using any built in sorting functions. The result must contain the same elements as the input, but arranged from smallest to largest. The input is an array nums containing integers. These...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1051 - Height Checker

LeetCode Problem 1051 Difficulty: 🟢 Easy Topics: Array, Sorting, Counting Sort Solution Problem Understanding The problem gives us an array called heights , where each value represents the height of a student standing in a line. The school wants the students arranged in non-decreasing order, meaning heights should appear from smallest to largest, allowing duplicates. We are asked to determine how many students are currently standing in the wrong position...

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 18xx ›

LeetCode 1833 - Maximum Ice Cream Bars

LeetCode Problem 1833 Difficulty: 🟡 Medium Topics: Array, Greedy, Sorting, Counting Sort Solution Problem Understanding This problem asks us to determine the maximum number of ice cream bars a boy can buy with a limited number of coins. The input consists of an array costs , where costs[i] represents the price of the i-th ice cream bar, and an integer coins representing the total coins available. The boy can purchase...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2037 - Minimum Number of Moves to Seat Everyone

LeetCode Problem 2037 Difficulty: 🟢 Easy Topics: Array, Greedy, Sorting, Counting Sort Solution Problem Understanding This problem asks us to assign students to seats such that every student occupies exactly one seat and no two students share the same seat. Each student can move left or right on a number line, and every movement by one position costs exactly one move. The goal is to minimize the total number of...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 561 - Array Partition

LeetCode Problem 561 Difficulty: 🟢 Easy Topics: Array, Greedy, Sorting, Counting Sort Solution Problem Understanding The problem gives an array nums containing 2n integers. Our task is to divide these integers into exactly n pairs. For each pair (ai, bi) , we take the smaller value, min(ai, bi) . After computing the minimum value from every pair, we sum all of those minimums together. The goal is to arrange the...

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 (archived) › LeetCode 10xx ›

LeetCode 1051: Height Checker

Problem Restatement We are given an array heights representing the current order of students. The expected order is the students sorted by height in non-decreasing order. Return the number of indices where the current height does not match the expected height. The official constraints state that 1 <= heights.length <= 100 and 1 <= heights[i] <= 100 . Input and Output Item Meaning Input Array heights Output Number of positions...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 791: Custom Sort String

Problem Restatement We are given two strings: order s All characters in order are unique. The string order defines a custom ordering of characters. We need to rearrange the characters of s so that characters appearing in order follow that same relative order. Characters in s that do not appear in order may be placed anywhere in the result. Return any valid permutation of s . Input and Output Item...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 274: H-Index

Problem Restatement We are given an integer array citations . Each value: citations[i] is the number of citations received by the researcher's i th paper. Return the researcher's h-index. The h-index is the maximum value h such that the researcher has at least h papers with at least h citations each. The constraints are 1 <= citations.length <= 5000 and 0 <= citations[i] <= 1000 . The official examples include...

Programming › Algorithm Cookbook › Chapter 6. Sorting ›

6.7 Counting Sort

6.7 Counting Sort Counting sort replaces comparisons with counting. It assumes keys come from a small integer range and uses that structure to achieve linear time. Instead of ordering elements directly, it counts how many times each key appears and reconstructs the sorted output from those counts. Problem You have an array A[0..n-1] where each element is an integer in a known range [0, k] . You want a stable...

Wiki › Algorithms › 01. Searching and Sorting › 8. Integer and Distribution Sorting ›

Key Indexed Counting

Key Indexed Counting Key indexed counting is a stable linear time sorting method for items with small integer keys. It generalizes counting sort by separating keys from records and placing records directly into their correct index range. This form is widely used in string sorting and radix-based pipelines. Problem Given an array $A$ of records, where each record has a key in range $[0, k)$, reorder the array so that...

Wiki › Algorithms › 01. Searching and Sorting › 8. Integer and Distribution Sorting ›

Stable Counting Sort

Stable Counting Sort Stable counting sort extends counting sort by preserving the relative order of equal elements. This property is required in multi-pass algorithms such as radix sort. The algorithm uses prefix sums to compute exact output positions for each element. Problem Given an array $A$ of $n$ elements with integer keys in range $[0, k]$, produce a sorted array while preserving the relative order of elements with equal keys....

Wiki › Algorithms › 01. Searching and Sorting › 11. Parallel, Distributed, and GPU Sorting ›

Parallel Counting Sort

Parallel Counting Sort Parallel counting sort extends counting sort by distributing counting and placement across multiple workers. It assumes keys are integers within a bounded range. Instead of comparing elements, it counts occurrences and reconstructs the sorted output. The algorithm consists of three main stages: local counting, prefix sum computation, and parallel scatter. Problem Given an array $A$ of size $n$ with integer keys in range $[0, k)$, produce a...

Wiki › Algorithms › 01. Searching and Sorting › 8. Integer and Distribution Sorting ›

Counting Sort

Counting Sort Counting sort is a non-comparison sorting algorithm for integer keys within a bounded range. It counts how many times each value appears, then reconstructs the sorted order using these counts. It runs in linear time when the key range is not significantly larger than the number of elements. Problem Given an array $A$ of $n$ integers where each value lies in a range $[0, k]$, produce a sorted...

Wiki › Algorithms › 01. Searching and Sorting › 8. Integer and Distribution Sorting ›

Sparse Key Counting Sort

Sparse Key Counting Sort Sparse key counting sort handles large key ranges by storing counts only for keys that appear in the input. It replaces the dense count array of counting sort with a map from key to frequency. This is useful when the key universe is huge but the number of distinct keys is small. Problem Given an array $A$ of $n$ keys drawn from a large domain, sort...

Wiki › Algorithms › 01. Searching and Sorting › 8. Integer and Distribution Sorting ›

Counting Sort with Negative Keys

Counting Sort with Negative Keys Counting sort normally assumes keys lie in a nonnegative range. This variant handles negative integers by shifting all values so that the minimum becomes zero, then applying standard counting sort. Problem Given an array $A$ of $n$ integers that may include negative values, sort the array. Let: $$ \min = \min(A),\quad \max = \max(A) $$ Idea Shift every value by $-\min$ so all keys become...

#counting-sort

LeetCode 3517 - Smallest Palindromic Rearrangement I

LeetCode 3088 - Make String Anti-palindrome

LeetCode 3189 - Minimum Moves to Get a Peaceful Board

LeetCode 274 - H-Index

LeetCode 912 - Sort an Array

LeetCode 1051 - Height Checker

LeetCode 1122 - Relative Sort Array

LeetCode 1833 - Maximum Ice Cream Bars

LeetCode 2037 - Minimum Number of Moves to Seat Everyone

LeetCode 561 - Array Partition

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

LeetCode 1051: Height Checker

LeetCode 791: Custom Sort String

LeetCode 274: H-Index

6.7 Counting Sort

Key Indexed Counting

Stable Counting Sort

Parallel Counting Sort

Counting Sort

Sparse Key Counting Sort

Counting Sort with Negative Keys