Need advice on code optimization

I am trying to generate the subsequences of two input string but my code is taking long to compile.Just need advice for optimizing the given code.Below is the code

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;


class Subsequences {

       public static void combinations(String suffix,String prefix,List seq){
        if(prefix.length()<0)return;

        seq.add(suffix);
        for(int i=0;i<prefix.length();i++)
         combinations(suffix+prefix.charAt(i),prefix.substring(i+1,prefix.length()),seq);
    }



    public static void main(String args[] ) throws Exception {

        List seq1=new ArrayList();
        List seq2=new ArrayList();
        boolean flag=false;
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String line = br.readLine();
        int N = Integer.parseInt(line);
        for (int i = 0; i < N; i++) {
        combinations("",br.readLine(),seq1);
        combinations("",br.readLine(),seq2,flag,seq1);
        if(flag){
            System.out.println("Yes");
        }else{
            System.out.println("No");
        }
        seq1.retainAll(seq2);
        if(seq1.size()>1){
            System.out.println("Yes");
        }else{
            System.out.println("No");
        }
        seq1.clear();
        seq2.clear();
        }


        System.out.println("Hello World!");
    }
}

Herein I am generating all the substrings of a string using recursion and storing all of these into arraylist.Then using retainAll I compare any common element between then.