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

        }
    }