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)

    此题和3sum思路基本相同，不同的是，需要固定两个数，而不是一个数，而且要求4个数的和是target而不是0。

1. class Solution {
   private:
       vector<vector<int>> result;
   public:
       vector<vector<int>> fourSum(vector<int>& nums, int target) {
           if(nums.size()<4)
               return result;
           sort(nums.begin(),nums.end());
           vector<int> temp(4);
           for(int i=0;i<nums.size()-3;i++){
               if(i!=0&&nums[i]==nums[i-1])
                   continue;
               for(int j=i+1;j<nums.size()-2;j++){
                   if(j!=i+1&&nums[j]==nums[j-1])
                       continue;
                   int left=j+1;
                   int right=nums.size()-1;
                   while(left<right){
                       int sum=nums[i]+nums[j]+nums[left]+nums[right];
                       if(sum==target){
                           temp[0]=nums[i];
                           temp[1]=nums[j];
                           temp[2]=nums[left];
                           temp[3]=nums[right];
                           result.push_back(temp);
                           left++;
                           right--;
                           while(left<right&&nums[left]==nums[left-1])
                               left++;
                           while(left<right&&nums[right]==nums[right+1])
                               right--;
                       }
                       else if(sum>target)
                           right--;
                       else 
                           left++;
                       }
                   }
               
           }
           return result;
       }
   };