Counting Sort

Counting Sort is an sorting algorithm, which sorts the integers( or Objects) given in a specific range.

Algorithm: Time Complexity O(n)

  • Take two arrays, Count[] and Result[] and given array is input[].
  • Count[] will store the counts of each integer in the given array.
  • Update the Count[] so that each index will store the sum till previous step. (Count[i]=Count[i] + Count[i-1]). Now updated Count[] array will reflect the actual position of each integer in Result[].
  • Now navigate the input array taking one element at a time, Count[input[i]] will tell you the index position of input[i] in Result[]. When you do that, decrease the count in Count[input[i]] by 1. See Example

Counting-Sort
Counting-Sort

Code:

import java.util.Arrays;
public class CountingSort {
public int[] sort(int[] A) {
int[] Result = new int[A.length + 1];
int[] Count = new int[A.length + 1];
for (int i = 0; i < Count.length; i++) {
Count[i] = 0; // put count for every element as 0
}
// Count[] will store the counts of each integer in the given array
for (int i = 0; i < A.length; i++) {
int x = Count[A[i]];
x++;
Count[A[i]] = x;
}
// • Update the Count[] so that each index will store the sum till
// previous step. (Count[i]=Count[i] + Count[i-1]).
// Now updated Count[] array will reflect the actual position of each
// integer in Result[].
for (int i = 1; i < Count.length; i++) {
Count[i] = Count[i] + Count[i 1];
}
// • Now navigate the input array taking one element at a time,
// Count[input[i]] will tell you the index position of input[i] in
// Result[]. When you do that, decrease the count in Count[input[i]] by
// 1.
for (int i = A.length 1; i >= 0; i) {
int x = Count[A[i]];
Result[x] = A[i];
x;
Count[A[i]] = x;
}
return Result;
}
public static void main(String[] args) {
// TODO Auto-generated method stub
int input[] = { 2, 1, 4, 5, 7, 1, 1, 8, 9, 10, 11, 14, 15, 3, 2, 4 };
System.out.println("Orginal Array " + Arrays.toString(input));
CountingSort c = new CountingSort();
int[] B = c.sort(input);
System.out.println("Sorted Array : " + Arrays.toString(B));
}
}

view raw
CountingSort.java
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Output :

Orginal Array [2, 1, 4, 5, 7, 1, 1, 8, 9, 10, 11, 14, 15, 3, 2, 4]
Sorted Array : [0, 1, 1, 1, 2, 2, 3, 4, 4, 5, 7, 8, 9, 10, 11, 14, 15]