Get all the permutations of a string in Java

Question

What is the solution for getting all the permutations of a given string using a recursive method

Answer 1

This is the recursive way of finding a solution for this program:

public static void permutation(String str) { 
    permutation("", str); 
}
private static void permutation(String pre, String str) {
    int num = str.length();
    if (num == 0) System.out.println(pre);
    else {
        for (int i = 0; i < num; i++)
            permutation(pre + str.charAt(i), str.substring(0, i) + str.substring(i+1, num));
    }
}

Answer 2

You could use recursion to do this.

•  Try each of the letters in turn as the first letter and then find all the permutations of the remaining letters using a recursive call.

•  The base case is when the input is an empty string the only permutation is the empty string

Here's how you could probably implement it:

public class PermutationsTest {
    public static void main(String[] args) throws Exception {
        String str = "sample";
        StringBuffer strBuf = new StringBuffer(str);
        doPerm(strBuf,0);
    }
    private static void doPerm(StringBuffer str, int index){
        if(index == str.length())
            System.out.println(str);            
        else { //recursively solve this by placing all other chars at current first pos
            doPerm(str, index+1);
            for (int i = index+1; i < str.length(); i++) {//start swapping all other chars with current first char
               swap(str,index, i);
               doPerm(str, index+1);
                swap(str,i, index);//restore back my string buffer
            }
        }
    }
    private  static void swap(StringBuffer str, int pos1, int pos2){
        char c1 = str.charAt(pos1);
        str.setCharAt(pos1, str.charAt(pos2));
        str.setCharAt(pos2, c1);
    }}