15. 3Sum

1. class Solution {  
2. public:  
3.     vector<vector<int> > threeSum(vector<int> &num) {    
4.         vector<vector<int> >  result;    
5.   
6.         sort(num.begin(), num.end());    
7.           
8.         /*Call 2 sum*/  
9.         for(int i = 0; i < num.size(); i++){    
10.             if(i > 0 && num[i] == num[i-1]) continue;    
11.             twoSum(num, result, i);  
12.         }   
13.         return result;    
14.     }  
15.       
16.     /*2 sum algorithm. different from the other one is that the other one return index not value*/  
17.     void twoSum(vector<int> &num, vector<vector<int>> &res, int targetIndex) {  
18.         /*starting from targetIndex + 1 is because the numbers before have been used*/  
19.             int i = targetIndex + 1;     
20.             int j = num.size()-1;    
21.               
22.             while(i < j){    
23.                 if(num[targetIndex] + num[i] + num[j] < 0)   i++;    
24.                 else if(num[targetIndex] + num[i] + num[j] > 0)   j--;    
25.                 else{    
26.                     vector<int> v;    
27.                     v.push_back(num[targetIndex]);    
28.                     v.push_back(num[i]);    
29.                     v.push_back(num[j]);    
30.                     res.push_back(v);    
31.                     i++; j--;    
32.                     while(i < num.size() && num[i]==num[i - 1]) i++;    
33.                     while(j >= 0 && num[j] == num[j + 1]) j--;    
34.                 }    
35.             }  
36.               
37.     }  
38. };