Leetcode: 3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

void threeSumHelper(vector<int> &num, int index, vector<int> &path, int cursum, vector<vector<int>> &res)    {    if(path.size() >=3){if(path.size() == 3 && cursum == 0){res.push_back(path);return;}elsereturn;}for(int i = index; i < num.size(); i++)    {path.push_back(num[i]);cursum += num[i];threeSumHelper(num,i+1,path,cursum,res);path.pop_back();cursum -= num[i];while(i < num.size()-1 && num[i] == num[i+1])i++;}}vector<vector<int> > threeSum(vector<int> &num) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<vector<int>> res;if(num.size()<3)return res;sort(num.begin(), num.end());vector<int> path;threeSumHelper(num,0,path,0,res);return res;    }

2sum變形:
vector<vector<int> > threeSum(vector<int> &num) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<vector<int>> res;if(num.size()<3)return res;set<vector<int>> tmpres;sort(num.begin(), num.end());for(int i = 0; i < num.size(); i++){int target = - num[i];int begin = i+ 1;int end = num.size()-1;while(begin < end){int sum = num[begin] + num[end];if( sum == target){vector<int> tmp;tmp.push_back(num[i]);tmp.push_back(num[begin]);tmp.push_back(num[end]);tmpres.insert(tmp);begin++;end--;}else if(sum < target)begin++;elseend--;}}set<vector<int>>::iterator it = tmpres.begin();for(; it != tmpres.end(); it++)res.push_back(*it);return res;    }