Leetcode: Subsets II

Given a collection of integers that might contain duplicates, S, return all possible subsets.

Note:
Elements in a subset must be in non-descending order.
The solution set must not contain duplicate subsets.
For example,
If S = [1,2,2], a solution is:

[
  [2],
  [1],
  [1,2,2],
  [2,2],
  [1,2],
  []
]

 1 import java.util.*;
 2 
 3 public class Solution {
 4     public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {
 5         java.util.Arrays.sort(num);
 6         ArrayList<Integer> set = new ArrayList<Integer>();
 7         ArrayList<ArrayList<Integer>> sets = new ArrayList<ArrayList<Integer>>();
 8         
 9         for (int i = 0; i <= num.length; i++) {
10             helper(set, sets, num, i, 0);
11         }
12         return sets;
13     }
14     
15     public void helper(ArrayList<Integer> set, ArrayList<ArrayList<Integer>> sets, int[] num, int i, int start) {
16         if (set.size() == i) {
17             if (!sets.contains(set)) {
18                 sets.add(new ArrayList<Integer>(set));
19             }
20             return;
21         }
22         for (int k = start; k < num.length; k++) {
23             set.add(num[k]);
24             helper(set, sets, num, i, k+1);
25             set.remove(set.size() - 1);
26         }
27     }
28 }