Your code here...
void search(int k) {
  if (k == n) {
    // process subset
  } else {
     search(k+1);
     subset.push_back(k);
     search(k+1);
     subset.pop_back();
   }
}

I am having a really hard time understand this code. I especially don't understand how 'k' turns to 2 after 'k' reaches 3. I think It's related to the thing backtracking. Someone plz help!!!