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 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.