Subsets with repeated elements

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],
  []
]

    public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        Arrays.sort(num);
        return help(num, 0);
    }
    
    public ArrayList<ArrayList<Integer>> help(int[] num, int index) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        Set<ArrayList<Integer>> set = new HashSet<ArrayList<Integer>>();
        if(index == num.length) {
            result.add(new ArrayList<Integer>());
            return result;
        }
        for(ArrayList<Integer> sub : help(num, index + 1)) {
            set.add(sub);
            ArrayList<Integer> a = new ArrayList<Integer>();
            a.add(num[index]);
            a.addAll(sub);
            if(!set.contains(a)) set.add(a);
        }
        result.addAll(set);
        return result;
    }