4Sum

  class Solution {
 public:
     vector<vector<int> > fourSum(vector<int> &num,int target) {
        
         int n = num.size();        
         sort(num.begin(), num.end());
         vector<vector<int> > res; 
         if(n<4) return res;
         for(int i = 0; i < n-2; i++)
         for(int j=i+1;j<n-1;j++)
         {            
             int target2 = target - num[i]-num[j];
             twoSum(num,i, j+1, target2, res);
         }
         sort(res.begin(),res.end());
         res.erase(unique(res.begin(),res.end()),res.end());
         return res;
     }
     void twoSum(vector<int> &sortedNum, int i,int start, int target, vector<vector<int> >&res)
     {
         int head = start, tail = sortedNum.size() - 1;
         while(head < tail)
         {
             int tmp = sortedNum[head] + sortedNum[tail];
             if(tmp < target)
                 head++;
             else if(tmp > target)
                 tail--;
             else
             {
                 res.push_back(vector<int>{sortedNum[i],sortedNum[start-1], sortedNum[head], sortedNum[tail]});
                 head++;
                 tail--;               
             }
         }
     }
 };