public List<List<Integer>> permuteUnique(int[] nums) {
        // 全排列
        Arrays.sort(nums);
        List<List<Integer>> resultList = new ArrayList<List<Integer>>();
        // DFS
        permuteDFS(resultList, nums, 0);
        
        return resultList;
    }
    
    private void permuteDFS(List<List<Integer>> resultList, int[] nums, int index) {
        
        if(index >= nums.length) {
            ArrayList<Integer> targetList = new ArrayList<Integer>();
            for(int i : nums)
                targetList.add(i);
            resultList.add(targetList);
            return;
        }
        Set<Integer> appeared = new HashSet<>();  
        for (int i = index; i < nums.length; i++) {
            if(appeared.add(nums[i])) {
                swap(nums, i, index);
                permuteDFS(resultList, nums, index+1);
                swap(nums, i, index);
            }
            //while(i < nums.length-1 && nums[i+1] == nums[i]) i++;
        }
        
    }

    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }