queue<TreeNode*> q;         // To store the node addresses
vector<int> result;         // To store the values in the node
result.push_back(root->val);
q.push(root);
while (!q.empty()) {
  node = q.front();
  q.pop();
  if(node->left){
    result.push_back(node->left->val);
    q.push(node->left);
  }else{
      result.push_back(NULL);
  }
  if(node->right){
    result.push_back(node->right->val);
    q.push(node->right);
  }else{
      result.push_back(NULL);
  }
}