## 题目地址

https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

## 题目描述

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]
]
``````

## 代码

``````class Solution {
public:
vector<vector<int> > levelOrderBottom(TreeNode* root) {
if (!root) return {};
vector<vector<int>> res;
queue<TreeNode*> q{{root}};
while (!q.empty()) {
vector<int> oneLevel;
for (int i = q.size(); i > 0; --i) {
TreeNode *t = q.front(); q.pop();
oneLevel.push_back(t->val);
if (t->left) q.push(t->left);
if (t->right) q.push(t->right);
}
res.insert(res.begin(), oneLevel);
}
return res;
}
};
``````

``````class Solution {
public:
vector<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> res;
levelorder(root, 0, res);
return vector<vector<int>> (res.rbegin(), res.rend());
}
void levelorder(TreeNode* node, int level, vector<vector<int>>& res) {
if (!node) return;
if (res.size() == level) res.push_back({});
res[level].push_back(node->val);
if (node->left) levelorder(node->left, level + 1, res);
if (node->right) levelorder(node->right, level + 1, res);
}
};
``````

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