Add Two Integers

LinkedListNode* add_integers(
    LinkedListNode* integer1, 
    LinkedListNode* integer2) {
  //TODO: Write - Your - Code
  return integer1;
}

// assuming both integers are stored in a linked list
// e.g. 415 is stored as 5->1->4
// 32 is stored as 2->3
LinkedListNode* add_integers(
    LinkedListNode* integer1, 
    LinkedListNode* integer2) {
  LinkedListNode* result = nullptr;
  LinkedListNode* last = nullptr;
  int carry = 0;
  
  while (
      integer1 != nullptr ||
      integer2 != nullptr ||
      carry > 0) {
    int first = 
        (integer1 == nullptr ? 0 : integer1->data);
    int second = 
        (integer2 == nullptr ? 0 : integer2->data);
    int sum = first + second + carry;
    LinkedListNode* pNew = 
          new LinkedListNode(sum % 10);
    
    carry = sum / 10;
    if (result == nullptr) {
      result = pNew;
    } else {
      last->next = pNew;
    }
    last = pNew;
    
    if (integer1 != nullptr) {
      integer1 = integer1->next;
    }
    
    if (integer2 != nullptr) {
      integer2 = integer2->next;
    }
  }
  
  return result;
}
int main(int argc, char* argv[]) {
	vector<int> v1 = {1, 2, 3}; // 321
  vector<int> v2 = {1, 2}; // 21
  
  LinkedListNode* first = LinkedList::create_linked_list(v1);
  LinkedListNode* second = LinkedList::create_linked_list(v2);
  // sum should be 321 + 21 = 342 => 2->4->3
  LinkedListNode* result = add_integers(first, second);
  vector<int> r = {2, 4, 3}; // 342
  LinkedListNode* expected = LinkedList::create_linked_list(r);
  assert(LinkedList::is_equal(result, expected));
  cout << endl << "First:";
  LinkedList::display(first);
  cout << endl << "Second:";
  LinkedList::display(second);
  cout << endl << "Result:";
  LinkedList::display(result);
  result = add_integers(first, nullptr);
  assert(LinkedList::is_equal(result, first));
  result = add_integers(nullptr, second);
  assert(LinkedList::is_equal(result, second));
}