题目：

题解：

class Solution {
public:vector<vector<int>> levelOrderBottom(TreeNode* root) {auto levelOrder = vector<vector<int>>();if (!root) {return levelOrder;}queue<TreeNode*> q;q.push(root);while (!q.empty()) {auto level = vector<int>();int size = q.size();for (int i = 0; i < size; ++i) {auto node = q.front();q.pop();level.push_back(node->val);if (node->left) {q.push(node->left);}if (node->right) {q.push(node->right);}}levelOrder.push_back(level);}reverse(levelOrder.begin(), levelOrder.end());return levelOrder;}
};