【LeetCode】16. 4Sum

题目：Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• The solution set must not contain duplicate quadruplets.

给定一个数组，找出所有4个数的组合使得它们的和为target

思路：和3Sum，Two Sum都是类似的。主要是两个点：

1. 对数组排序，然后双指针分别从头尾遍历

2. 注意去重的问题

 class Solution {
 public:
     vector<vector<int>> fourSum(vector<int>& nums, int target) {
         int nsize = nums.size();
         sort(nums.begin(), nums.end());
         vector<vector<int>> result;
         for(int i = ; i < nsize; i++){
             if(i !=  && nums[i] == nums[i - ])
                 continue;
             for(int j = i + ; j < nsize; j++){
                 if(j != i +  && nums[j] == nums[j - ])
                     continue;
                 int p = j + , q = nsize - ;
                 while(p < q){
                     int sum = nums[i] + nums[j] + nums[p] + nums[q];
                     if(sum < target){
                         ++p;
                         continue;
                     }
                     if(sum > target){
                         --q;
                         continue;
                     }

                         vector<int> tmp;
                         tmp.push_back(nums[i]);
                         tmp.push_back(nums[j]);
                         tmp.push_back(nums[p]);
                         tmp.push_back(nums[q]);
                         result.push_back(tmp);

                         while(++p < q && nums[p] == nums[p - ]);
                         while(p < --q && nums[q] == nums[q + ]);
                 }
             }
         }
         return result;
     }
 };