key

1.sort array

2.anchor one num, traverse others

tip:check the duplicated nums

comapre with 2sum

vector<vector<int> > threeSum(vector<int> &num) {
        sort(num.begin(),num.end());
        vector<vector<int> > res;
        for(int i =0 ; i < num.size(); i++){
            int target = -num[i];
            int start = i+1;
            int end = num.size() -1;
            while(start < end){
                int sum = num[start]+num[end];
                if(sum > target) end--;
         
                else if(sum < target) start++;
                
                 else{    
                        vector<int> temp(3,0);
                        temp[0] = num[i];
                        temp[1] = num[start];
                        temp[2] = num[end];
                        res.push_back(temp);
                        
                        while(start < end && num[start] == temp[1]) start++;
                        while(start < end && num[end] == temp[2]) end--;
                }
            }
            while(i+1 < num.size() && num[i] == num[i+1])
                i++;
        }
        return res;