public List<List<Integer>> fourSum(int[] nums, int target) {

         List<List<Integer>> res = new ArrayList<>();

         Arrays.sort(nums);

         for (int i = 0; i < nums.length - 3; i++) {

             if (i != 0 && nums[i] == nums[i - 1])

                 continue;

             for (int j = i + 1; j < nums.length - 2; j++) {

                 if (j > i + 1 && nums[j] == nums[j - 1])

                     continue;

                 int k = j + 1;

                 int l = nums.length - 1;

                 while (k < l) {

                     int sum = nums[i] + nums[j] + nums[k] + nums[l];

                     if (sum == target) {

                         List<Integer> list = new ArrayList<>();

                         list.add(nums[i]);

                         list.add(nums[j]);

                         list.add(nums[k]);

                         list.add(nums[l]);

                         res.add(list);

                         k++;

                         l--;

                         // 去重复

                         while (k < l && nums[k] == nums[k - 1]) {

                             k++;

                         }

                         while (k < l && nums[l] == nums[l + 1]) {

                             l--;

                         }

                     } else if (sum < target) {

                         k++;

                     } else {

                         l--;

                     }

                 }

             }

         }

         return res;

     }

 public List<List<Integer>> fourSum3(int[] num, int target) {

         ArrayList<List<Integer>> ans = new ArrayList<>();

         if (num.length < 4)

             return ans;

         Arrays.sort(num);

         for (int i = 0; i < num.length - 3; i++) {

             if (num[i] + num[i + 1] + num[i + 2] + num[i + 3] > target)

                 break; // first candidate too large, search finished

             if (num[i] + num[num.length - 1] + num[num.length - 2] + num[num.length - 3] < target)

                 continue; // first candidate too small

             if (i > 0 && num[i] == num[i - 1])

                 continue; // prevents duplicate result in ans list

             for (int j = i + 1; j < num.length - 2; j++) {

                 if (num[i] + num[j] + num[j + 1] + num[j + 2] > target)

                     break; // second candidate too large

                 if (num[i] + num[j] + num[num.length - 1] + num[num.length - 2] < target)

                     continue; // second candidate too small

                 if (j > i + 1 && num[j] == num[j - 1])

                     continue; // prevents duplicate results in ans list

                 int low = j + 1, high = num.length - 1;

                 while (low < high) {

                     int sum = num[i] + num[j] + num[low] + num[high];

                     if (sum == target) {

                         ans.add(Arrays.asList(num[i], num[j], num[low], num[high]));

                         while (low < high && num[low] == num[low + 1])

                             low++; // skipping over duplicate on low

                         while (low < high && num[high] == num[high - 1])

                             high--; // skipping over duplicate on high

                         low++;

                         high--;

                     }

                     // move window

                     else if (sum < target)

                         low++;

                     else

                         high--;

                 }

             }

         }

         return ans;

     }