vector<vector<int> > threeSum(vector<int> &num) {

vector <vector<int> > result;

size_t n = num.size();

if(n < 3) return result;

vector<int> solution(3);

sort(num.begin(), num.end());

for(int i = 0; i < n - 2; ++i) {

int start = i + 1, end = n - 1;

while(start < end) {

int sum = num[start] + num[end] + num[i];

if(sum == 0) {

solution.at(0) = num[i];

solution.at(1) = num[start];

solution.at(2) = num[end];

result.push_back(solution);

// these loops will skip same elements and fasten the algorithm

// while(start < n - 1 and num[start] == num[start + 1]) start++;

// while(end > i + 1 and num[end] == num[end - 1]) end--;

start++, end--;

} else if(sum > 0) {

// while(end > i + 1 and num[end] == num[end - 1]) end--;

end--;

} else {

// while(start < n - 1 and num[start] == num[start + 1]) start++;

start++;

}

}

while(i < n - 1 and num[i] == num[i + 1]) i++;

}

return result;

}