Method permuteUnique

java/0047-permutations-ii.java:2–8

(int[] nums)

class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();

        backtrack(res, nums, new ArrayList<>(), new boolean[nums.length]);
        return res;
    }

    public void backtrack(List<List<Integer>> res,
                          int[] nums,