LeetCode 107. Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:

Given binary tree

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its bottom-up level order traversal as:

[
[15,7],
[9,20],
[3]
]

Nothing special....

vector<vector<int>> levelOrderBottom(TreeNode* root) {
if(!root) return {};
vector<TreeNode*> nodes;
nodes.push_back(root);
vector<vector<int>> res;
while(!nodes.empty()) {
vector<int> path;
vector<TreeNode*> nextLevel;
for(int i = 0; i < nodes.size(); ++i) {
path.push_back(nodes[i]->val);
if(nodes[i]->left) nextLevel.push_back(nodes[i]->left);
if(nodes[i]->right) nextLevel.push_back(nodes[i]->right);
}
res.push_back(path);
path.clear();
nodes.swap(nextLevel);
nextLevel.clear();
}
reverse(res.begin(), res.end());
return res;
}