4sum

k-sum问题都可以有2-sum引申出来解决，但是时间复杂度为O(n^k-1)。应该要用哈希解决才好的，之后再看看

class Solution{
private:
  vector<vector<int> > ans;
public:
  vector<vector<int> >fourSum(vector<int> &num, int target){
    sort(num.begin(),num.end());
    ans.clear();
    for(int i = 0; i < num.size(); ++i) {
      if(i > 0 && num[i] == num[i-1]) continue;
      for(int j = i+1; j < num.size(); ++j){
          if(j > i+1 && num[j] == num[j-1]) continue;
          int k = j+1;
          int t = num.size()-1;
          while(k < t){
            if(k > j+1 && num[k] == num[k-1]){
              k++;
              continue;
            }
            if(t < num.size()-1 && num[t] == num[t+1]){
              t--;
              continue;
            }
            int sum = num[i]+num[j]+num[k]+num[t];
            if(sum == target){
              vector<int>a;
              a.push_back(num[i]);
              a.push_back(num[j]);
              a.push_back(num[k]);
              a.push_back(num[t]);
              ans.push_back(a);
              k++;
            }
            else if(sum < target)
              k++;
            else 
              t--;
          } 
      }
    }
    return ans;
  }
};