LeetCode_4Sum

 1 class Solution {

 2 public:

 3     vector<vector<int> > fourSum(vector<int> &num, int target) {

 4         // Start typing your C/C++ solution below

 5         // DO NOT write int main() function

 6          vector<vector<int> > output; 

 7          if(num.size()<4) return output; 

 8          

 9          int sum;

10          sort(num.begin(), num.end());

11          

12          for(int i =0; i< num.size() -3 ;){

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

14                 int startS = j + 1 ;

15                 int endS = num.size() - 1 ;

16                 

17                 while(startS < endS){

18               

19                     sum = num[i] + num[j] + num[startS] + num[endS];

20                     if(sum == target) {

21                         vector<int> triplet;  

22                         triplet.push_back(num[i]); 

23                         triplet.push_back(num[j]); 

24                         triplet.push_back(num[startS]);  

25                         triplet.push_back(num[endS]);  

26                         output.push_back(triplet);  

27                         startS++ ;

28                         while(num[startS] == num[startS-1] && startS < endS) startS++;

29                     }

30                     if(sum < target)

31                         startS ++;

32                     else

33                             endS --;    

34                 }   

35                 j++ ;

36                 while(num[j] == num[j-1] && j < num.size() -2 ) j++ ;

37              }

38              i++;

39              while(num[i] == num[i-1] && i< num.size() -3) i ++;     

40          }        

41          return output;

42     }

43 };

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同前面的3sum 差不多