3sum

双指针法

class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
        sort(num.begin(),num.end());
        int len = num.size();
        vector<vector<int>> ans;
        vector<int> tmp;
        for (int i=0;i<len;i++) {
           if (i>0&&num[i]==num[i-1]) continue;
              int j = i+1;
              int k = len-1;
                while (j<k) {
                 if (j>i+1&&num[j]==num[j-1]) {j++;continue;}  // two
                 if  (k<len-1&&num[k]==num[k+1]) {k--;continue;}//two  j===len k==len-1
                  int sum = num[i]+num[j]+num[k];
                    if(sum==0) {
                         tmp.clear();
                        tmp.push_back(num[i]);
                        tmp.push_back(num[j]);
                        tmp.push_back(num[k]);
                         ans.push_back(tmp);
                        j++;
                    }
                    else if(sum<0) j++;
                         else k--;
                }

            }
        return ans;
        }


};