Given a list of numbers that may has duplicate numbers, return all possible subsets

class Solution {
    /** * @param nums: A set of numbers. * @return: A list of lists. All valid subsets. */
    public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] nums) {
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        if (nums == null || nums.length == 0) {
            return results;
        }
        ArrayList<Integer> subset = new ArrayList<>();
        Arrays.sort(nums);
        dfsHelp(nums, 0, subset, results);
        return results;
    }
    private void dfsHelp(int[] nums,
                         int index,
                         ArrayList<Integer> subset,
                         ArrayList<ArrayList<Integer>> results) {
        results.add(new ArrayList<Integer>(subset));
        for (int i = index; i < nums.length; i++) {
            if (i != index && nums[i] == nums[i - 1]) {
                continue;
            }
            subset.add(nums[i]);
            dfsHelp(nums, i + 1, subset, results);
            subset.remove(subset.size() - 1);
        }
    }    
}