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--;                            }         }     } };