Smart way to generate permutation and combination of String

Question

String database[] = {\'a\', \'b\', \'c\'};

I would like to generate the following strings sequence, based on given database.

Answer 1

// IF YOU NEED REPEATITION USE ARRAYLIST INSTEAD OF SET!!

import java.util.*;
public class Permutation {

    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        System.out.println("ENTER A STRING");
        Set se=find(in.nextLine());
        System.out.println((se));
    }
    public static Set find(String s)
    {
        Set ss=new HashSet();
        if(s==null)
        {
            return null;
        }
        if(s.length()==0)
        {
            ss.add("");
        }
        else
        {
            char c=s.charAt(0);
            String st=s.substring(1);
            Set qq=find(st);
            for(String str:qq)
            {
                for(int i=0;i<=str.length();i++)
                {
                    ss.add(comb(str,c,i));
                }
            }
        }
        return ss;

    }
    public static String comb(String s,char c,int i)
    {
        String start=s.substring(0,i);
        String end=s.substring(i);
        return start+c+end;
    }

}


// IF YOU NEED REPEATITION USE ARRAYLIST INSTEAD OF SET!!