Given a collection of numbers that might contain duplicates, return all possible unique permutations.

class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if(nums == null || nums.length == 0)
            return res;

        Arrays.sort(nums);
        boolean[] visited = new boolean[nums.length];
        helper(res, new ArrayList<Integer>(), nums, visited);

        return res;
    }

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

        for(int i = 0; i < nums.length; i++) {
            if(visited[i])
                continue;

            if(i > 0 && nums[i] == nums[i - 1] && !visited[i - 1])
                continue;

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