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
// of a given string.
using System;

class GFG
{
    /* Permutation function @param
       str string to calculate permutation
       for @param l starting index @param
       r end index */
    private static void permute(String str,
                                int l, int r)
    {
        if (l == r)
            Console.WriteLine(str);
        else
        {
            for (int i = l; i <= r; i++)
            {
                str = swap(str, l, i);
                permute(str, l + 1, r);
                str = swap(str, l, i);
            }
        }
    }

    /* Swap Characters at position
       @param a string value @param
       i position 1 @param j position 2
       @return swapped string */
    public static String swap(String a,
                            int i, int j)
    {
        char temp;
        char[] charArray = a.ToCharArray();
        temp = charArray[i] ;
        charArray[i] = charArray[j];
        charArray[j] = temp;
        string s = new string(charArray);
        return s;
    }

// Driver Code
public static void Main()
{
    String str = "ABC";
    int n = str.Length;
    permute(str, 0, n-1);
}
}
// This code is contributed by mits

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

Another approach:

C#

// C# program to implement
// the above approach
using System;
public class GFG{

static void permute(String s,
                    String answer)
{
    if (s.Length == 0)
    {
        Console.Write(answer + "  ");
        return;
    }

    for(int i = 0 ;i < s.Length; i++)
    {
        char ch = s[i];
        String left_substr = s.Substring(0, i);
        String right_substr = s.Substring(i + 1);
        String rest = left_substr + right_substr;
        permute(rest, answer + ch);
    }
}

// Driver code
public static void Main(String []args)
{
    String s;
    String answer="";

    Console.Write(
    "Enter the string : ");
    s = Console.ReadLine();

    Console.Write(
    "\nAll possible strings are : ");
    permute(s, answer);
}
}
// This code is contributed by gauravrajput1

Output:

Enter the string : abc
All possible strings are : abc  acb  bac  bca  cab  cba

Time Complexity: O(n*n!) The time complexity is the same as the above approach, i.e. there are n! permutations and it requires O(n) time to print a permutation.

Auxiliary Space: O(|s|)

Please refer complete article on Write a program to print all permutations of a given string for more details!