Combination 알고리즘

[asd(121.187)]

package asdasd;
import java.util.ArrayList;
import java.util.Collections;

import javax.swing.*;

public class Combinations {
 public static ArrayList<String> al = new ArrayList<String>();

    public static void comb1(String s) { comb1("", s); }

    private static void comb1(String prefix, String s) {
        if (s.length() > 0) {
            al.add(prefix + s.charAt(0));
            comb1(prefix + s.charAt(0), s.substring(1));
            comb1(prefix,               s.substring(1));
        }
    } 

    public static void main(String[] args) {
       String alphabet = JOptionPane.showInputDialog("Enter sentence");
       String N = JOptionPane.showInputDialog("Enter number");
       String elements = alphabet.substring(0, Integer.parseInt(N));

       comb1(elements);

      
       Collections.sort(al);
       StringBuffer sb = new StringBuffer();
       for(int i=0; i<Integer.parseInt(N); i++) {
        for(int j=0; j<al.size(); j++) {
         if(al.get(j).length()==i+1) {
          sb.append("\""+al.get(j)+"\""+" ");
         }
        }
        sb.append("\n");
       }
       JOptionPane.showMessageDialog(null,sb.toString());
      
    }

}