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

vector<vector<int> > ret;

if (num.empty()) return ret;

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

int n = num.size();

vector<int> triplet(3);

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

if (i >0&& num[i] == num[i-1]) continue;

int j = i+1;

int k = n-1;

while (j <k) {

int sum = num[i] + num[j] + num[k];

if (sum ==0) {

triplet[0] = num[i];

triplet[1] = num[j];

triplet[2] = num[k];

ret.push_back(triplet);

j ++;

k --;

//understand this two blocks

while (j <k && num[j] == num[j-1]) j++;

while (j <k &&num[k] == num[k+1]) k--;

}

else if(sum <0) {

j ++;

}

else {

k --;

}

}

}

return ret;

}