C Program to print all permutations of a given string

A permutation also called an “arrangement number” or “order,” is a rearrangement of the elements of an ordered list S into a one-to-one correspondence with S itself. A string of length n has n! permutation. 

Source: Mathword(http://mathworld.wolfram.com/Permutation.html)

Below are the permutations of string ABC. 
ABC ACB BAC BCA CBA CAB

Here is a solution that is used as a basis in backtracking.

C

// C program to print all permutations
// with duplicates allowed
#include <stdio.h>
#include <string.h>

/* Function to swap values at
   two pointers */
void swap(char *x, char *y)
{
    char temp;
    temp = *x;
    *x = *y;
    *y = temp;
}

/* Function to print permutations
   of string
   This function takes three parameters:
   1. String
   2. Starting index of the string
   3. Ending index of the string. */
void permute(char *a, int l, int r)
{
    int i;
    if (l == r)
        printf("%s\n", a);
    else
    {
        for (i = l; i <= r; i++)
        {
            swap((a + l), (a + i));
            permute(a, l + 1, r);

            //backtrack
            swap((a + l), (a + i));
        }
    }
}

// Driver code
int main()
{
    char str[] = "ABC";
    int n = strlen(str);
    permute(str, 0, n-1);
    return 0;
}

Output:

ABC
ACB
BAC
BCA
CBA
CAB

Algorithm Paradigm: Backtracking 

Time Complexity: O(n*n!) Note that there are n! permutations and it requires O(n) time to print a permutation.

Auxiliary Space: O(r – l)

Note: The above solution prints duplicate permutations if there are repeating characters in the input string. Please see the below link for a solution that prints only distinct permutations even if there are duplicates in input.
Print all distinct permutations of a given string with duplicates.
Permutations of a given string using STL

Please write comments if you find the above codes/algorithms incorrect, or find other ways to solve the same problem.