Leetcode 47. Permutations II

public class Solution {
    public static void backTrack(int[] nums, boolean[] used, List<Integer> tmp, List<List<Integer>> res) {
        if (tmp.size() == nums.length) {
            res.add(new ArrayList<>(tmp));
            return;
        }
        else {
            for (int i=0; i<nums.length; i++) {
                if (used[i] || i>0 && nums[i-1]==nums[i] && !used[i-1]) continue;
                used[i] = true;
                tmp.add(nums[i]);
                backTrack(nums, used, tmp, res);
                used[i] = false;
                tmp.remove(tmp.size()-1);
            }
        }
    }
    
    public List<List<Integer>> permuteUnique(int[] nums) {
        Arrays.sort(nums);      // always sort when there are duplicates in the array
        List<List<Integer>> res = new ArrayList<>();
        backTrack(nums, new boolean[nums.length], new ArrayList<>(), res);
        return res;
    }
}