LeetCode_4Sum

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
         vector<vector<int> > output; 
         if(num.size()<4) return output; 
         
         int sum;
         sort(num.begin(), num.end());
         
         for(int i =0; i< num.size() -3 ;){
           for(int j = i+ 1; j < num.size() - 2; ){
                int startS = j + 1 ;
                int endS = num.size() - 1 ;
                
                while(startS < endS){
              
                    sum = num[i] + num[j] + num[startS] + num[endS];
                    if(sum == target) {
                        vector<int> triplet;  
                        triplet.push_back(num[i]); 
                        triplet.push_back(num[j]); 
                        triplet.push_back(num[startS]);  
                        triplet.push_back(num[endS]);  
                        output.push_back(triplet);  
                        startS++ ;
                        while(num[startS] == num[startS-1] && startS < endS) startS++;
                    }
                    if(sum < target)
                        startS ++;
                    else
                            endS --;    
                }   
                j++ ;
                while(num[j] == num[j-1] && j < num.size() -2 ) j++ ;
             }
             i++;
             while(num[i] == num[i-1] && i< num.size() -3) i ++;     
         }        
         return output;
    }
};

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同前面的3sum 差不多