leetcode——18——4Sum

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.

For example, given array S = {1 0 -1 0 -2 2}, and target = 0. A solution set is: (-1, 0, 0, 1) (-2, -1, 1, 2) (-2, 0, 0, 2)

class Solution {public:    vector<vector<int>> fourSum(vector<int>& nums, int target) {         vector<vector<int>> out;           set<vector<int>> res;         if (nums.size() < 4)              return out;          sort(nums.begin(), nums.end());          for (int i=0; i<nums.size()-3; i++)          {                            for (int j =i+1;j<nums.size()-2;j++)              {                  int begin = j+1;                  int end = nums.size()-1;                  while(begin < end){                      int sum = nums[i] + nums[j] + nums[begin] + nums[end];                      if (sum== target)                      {                          vector<int> cur(4);                        cur[0] = nums[i];                        cur[1] = nums[j];                        cur[2] = nums[begin];                        cur[3] = nums[end];                        res.insert(cur);                          begin++;                          end--;                      }                       else if(sum < target)                      {                          begin++;                      }else{                          end--;                      }                  }                                }          }          set<vector<int>>::iterator it = res.begin();            for(; it != res.end(); it++)                out.push_back(*it);               return out;    }};