Credit: http://www.geeksforgeeks.org/counting-sort/
Counting sort is a sorting technique based on keys between a specific range. It works by counting the number of objects having distinct key values (kind of hashing). Then doing some arithmetic to calculate the position of each object in the output sequence.
Let us understand it with the help of an example.
For simplicity, consider the data in the range 0 to 9.
Input data: 1, 4, 1, 2, 7, 5, 2
1) Take a count array to store the count of each unique object.
Index: 0 1 2 3 4 5 6 7 8 9
Count: 0 2 2 0 1 1 0 1 0 0
2) Modify the count array such that each element at each index
stores the sum of previous counts.
Index: 0 1 2 3 4 5 6 7 8 9
Count: 0 2 4 4 5 6 6 7 7 7
The modified count array indicates the position of each object in
the output sequence.
3) Output each object from the input sequence followed by
decreasing its count by 1.
Process the input data: 1, 4, 1, 2, 7, 5, 2. Position of 1 is 2.
Put data 1 at index 2 in output. Decrease count by 1 to place
next data 1 at an index 1 smaller than this index.
Time Complexity: O(n+k) where n is the number of elements in input array and k is the range of input.
#include <stdio.h>
#include <string.h>
#define RANGE 255
void countSort(char *str)
{
char output[strlen(str)];
// Create a count array to store count of inidividul characters and
// initialize count array as 0
int count[RANGE + 1], i;
memset(count, 0, sizeof(count));
// Store count of each character
for(i = 0; str[i]; ++i)
++count[str[i]];
// Change count[i] so that count[i] now contains actual position of
// this character in output array
for (i = 1; i <= RANGE; ++i)
count[i] += count[i-1];
// Build the output character array
for (i = 0; str[i]; ++i)
{
output[count[str[i]]-1] = str[i];
--count[str[i]];
}
// Copy the output array to str, so that str now
// contains sorted characters
for (i = 0; str[i]; ++i)
str[i] = output[i];
}
int main()
{
char str[] = "queryhome";
countSort(str);
printf("Sorted string is %s\n", str);
return 0;
}
Output
Sorted string is eehmoqruy
