1 public class Solution { 2     public List<List<Integer>> permute(int[] num) { 3         ArrayList<List<Integer>> result = new ArrayList<List<Integer>>(); 4         ArrayList<Integer> row = null; 5         if(num == null ||num.length == 0) return result; 6         long size = (long)Math.pow(2, num.length); 7         for(long i = 0; i < size; i ++){ 8             row = new ArrayList<Integer>(); 9             for(int j = 0; j < num.length; j ++){10                 if((i >> j) % 2 == 1) row.add(num[j]);11             }12             result.add(row);13         }14         return result;15     }16 }