Subclasses

　　Given a collection of numbers, return all possible subclasses.

 public class Solution {
     public List<List<Integer>> permute(int[] num) {
         ArrayList<List<Integer>> result = new ArrayList<List<Integer>>();
         ArrayList<Integer> row = null;
         if(num == null ||num.length == 0) return result;
         long size = (long)Math.pow(2, num.length);
         for(long i = 0; i < size; i ++){
             row = new ArrayList<Integer>();
             for(int j = 0; j < num.length; j ++){
                 if((i >> j) % 2 == 1) row.add(num[j]);
             }
             result.add(row);
         }
         return result;
     }
 }