Finding all the unique permutations of a string without generating duplicates

Question

Finding all the permutations of a string is by a well known Steinhaus–Johnson–Trotter algorithm. But if the string contains the repeated characters such as
AABB,
the

Answer 1

This is a tricky question and we need to use recursion to find all the permutations of a String, for example "AAB" permutations will be "AAB", "ABA" and "BAA". We also need to use Set to make sure there are no duplicate values.

import java.io.*;
import java.util.HashSet;
import java.util.*;
class Permutation {

    static HashSet set = new HashSet();
    public static void main (String[] args) {
    Scanner in = new Scanner(System.in);
        System.out.println("Enter :");
        StringBuilder  str = new StringBuilder(in.nextLine());
        NONDuplicatePermutation("",str.toString());  //WITHOUT DUPLICATE PERMUTATION OF STRING
        System.out.println(set);
    }


    public static void NONDuplicatePermutation(String prefix,String str){
        //It is nlogn
        if(str.length()==0){
            set.add(prefix);
        }else{
            for(int i=0;i