Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set).

Note:The solution set must not contain duplicate subsets.

class Solution {
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();

        if(nums == null) {
            res.add(new ArrayList<Integer>());
            return res;
        }

        boolean[] visited = new boolean[nums.length];
        Arrays.sort(nums);

        for(int i = 0; i <= nums.length; i++) 
            helper(res, new ArrayList<Integer>(), nums, visited, 0, i);

        return res;
    }

    public void helper(List<List<Integer>> res, List<Integer> line, int[] nums, boolean[] visited, int start, int size) {
        if(line.size() == size) {
            res.add(new ArrayList<>(line));
            return;
        }

        for(int i = start; i < nums.length; i++) {
            if(i > 0 && nums[i] == nums[i - 1] && !visited[i - 1])
                continue;

            line.add(nums[i]);
            visited[i] = true;
            helper(res, line, nums, visited, i + 1, size);
            visited[i] = false;
            line.remove(line.size() - 1);
        }
    }
}