Permutations II

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

• Time: O(n) Space: O(1)

public List<List<Integer>> permuteUnique(int[] nums) {
    List<List<Integer>> ret = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    Arrays.sort(nums);
    boolean[] flag = new boolean[nums.length];
    permuteUniqueDfs(ret, path, nums, flag);
    return ret;
}
private void permuteUniqueDfs(List<List<Integer>> ret, 
      List<Integer> path, int[] nums, boolean[] flag) {
    if (path.size() == nums.length) {
        ret.add(new ArrayList<>(path));
        return;
    }
    for (int i = 0; i < nums.length; i++) {
        if (!flag[i]) {
            if (i > 0 && nums[i] == nums[i - 1] && !flag[i - 1]) 
                continue;
            flag[i] = true;
            path.add(nums[i]);
            permuteUniqueDfs(ret, path, nums, flag);
            path.remove(path.size() - 1);
            flag[i] = false;
        }
    }
}