C Program for Reverse a linked list

C Program to print all permutations of a given string

Last Updated : 10 Dec, 2021

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

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

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

NewPermutation

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.

C Program for Reverse a linked list

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