void levelOrderHelper(TreeNode* root, int level, vector<vector<int>> &res) {
if(root == NULL) return;
if(res.size() == level)
res.push_back(vector<int>{});
res[level].push_back(root->val);
levelOrderHelper(root->left, level+1, res);
levelOrderHelper(root->right, level+1, res);
}
vector<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> res;
if(root == NULL) return res;
levelOrderHelper(root, 0, res);
return vector<vector<int>> {res.rbegin(), res.rend()};
}
void Tree<NODETYPE>::levelOrderTraversal() const {
levelOrderHelper(rootPtr);
std::cout << std::endl;
} // end method levelOrderTraversal
