LeetCode: 3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• The solution set must not contain duplicate triplets.

    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)

   typedef vector<vector<int> > vvi;  
   typedef vector<int> vi;  
   
   #define tr(v, i) for(decltype((v).begin()) (i) = (v).begin(); (i) != (v).end(); (i)++)  
   
   vector<vector<int> > threeSum(vector<int> &num) {  
     set< vector<int> > S;  
     vvi V;  
   
     sort(num.begin(),num.end());  
   
     int n = num.size();  
   
     int i, j,k,sum;  
     for (k = 0; k<n-2; k++)  
     {  
       i = k+1; j = n-1;  
       while(j > i)  
       {  
         sum = num[i]+num[j]+num[k];  
         if(sum == 0)  
         {  
           vi v;  
           v.push_back(num[k]);  
           v.push_back(num[i]);  
           v.push_back(num[j]);  
           S.insert(v);  
           i++;  
           j--;  
         }  
         else if (sum > 0)  
         {  
           j--;  
         }  
         else i++;  
       }  
     }  
   
     tr (S,it)  
     {  
       V.push_back(*it);  
     }  
   
     return V;  
   
   }