47. Permutations II

 1 class Solution {
 2     List<List<Integer>> res = new ArrayList<>();
 3     public List<List<Integer>> permuteUnique(int[] nums) {
 4         Arrays.sort(nums);
 5         backtrack(nums, new ArrayList<>(), new boolean[nums.length]);
 6         return res;
 7     }
 8     
 9     public void backtrack(int[] nums, List<Integer> list, boolean[] used) {
10         if(list.size() == nums.length) {
11             res.add(new ArrayList<>(list));
12         }
13         for(int i = 0; i < nums.length; i++) {
14             if(used[i] || (i > 0 && nums[i-1] == nums[i] && !used[i-1])) continue;
15             used[i] = true;
16             list.add(nums[i]);
17             backtrack(nums, list, used);
18             list.remove(list.size() - 1);
19             used[i] = false;
20         }
21     }   
22 }