LeetCode: 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)

   typedef vector< vector<int> > vvi;  
   typedef vector< int > vi;  
   
   #define tr(v, it) for(decltype((v).begin()) (it) = (v).begin(); (it) != (v).end(); (it)++)   
   
   vector<vector<int> > fourSum(vector<int> &num, int target) {  
     set < vector<int> > S;  
     vvi V;  
     sort(num.begin(),num.end());  
       
     int n = num.size();  
       
     for (int i = 0; i < n-3; i++)  
     {  
       for (int j = i+1; j <n-2; j++)  
       {  
         int k = j+1, l = n-1;  
         while(k<l)  
         {  
           int sum = num[i]+num[j]+num[k]+num[l];  
           if(sum == target)  
           {  
             vi v;  
             v.push_back(num[i]);  
             v.push_back(num[j]);  
             v.push_back(num[k]);  
             v.push_back(num[l]);  
             S.insert(v);  
             k++;  
             l--;  
           }  
           else if (sum<target)k++;  
           else l--;  
         }  
       }  
     }  
     tr(S,it)  
     {  
       V.push_back(*it);  
     }  
     return V;  
   }