String Permutation using Recursion

Lets see simple program to print all Permutations of given string using Recursion call.

import java.util.TreeSet;

public class Permutation {

 static TreeSet<String> combination = new TreeSet<String>();

 public static void main(String[] args) {
  String str = "ABCD";
  permutation("", str);
  printAllCombination();
 }

 private static void permutation(String prefix, String str) {
  int len = str.length();
  if (len == 0) {
   combination.add(prefix);
  } else {
   for (int i = 0; i < len; i++) {

    String p = prefix + str.charAt(i);
    String s = str.substring(0, i) + str.substring(i + 1, len);

    permutation(p, s);
   }
  }
 }

 private static void printAllCombination() {
  int index = 0;
  for (String string : combination) {
   System.out.println(++index + "\t : " + string);
  }
 }
}

OUTPUT:

1  : ABCD
2  : ABDC
3  : ACBD
4  : ACDB
5  : ADBC
6  : ADCB
7  : BACD
8  : BADC
9  : BCAD
10  : BCDA
11  : BDAC
12  : BDCA
13  : CABD
14  : CADB
15  : CBAD
16  : CBDA
17  : CDAB
18  : CDBA
19  : DABC
20  : DACB
21  : DBAC
22  : DBCA
23  : DCAB
24  : DCBA