**一. 題目描述** 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 ~~~ **二. 題目分析** 由于使用了vector，這一題只需Binary Tree Level Order Traversal的基礎上加一句reverse(result.begin(), result.end()) 即可。印象中編程之美上有這題。 **三. 示例代碼** ~~~ #include <iostream> #include <vector> using namespace std; struct TreeNode { int val; TreeNode *left; TreeNode *right; TreeNode(int x) :val(x), left(NULL), right(NULL){} }; class Solution { public: vector<vector<int> > levelOrderBottom(TreeNode *root) { vector<vector<int> > result; orderTraversal(root, 1, result); reverse(result.begin(), result.end()); // 新增 return result; } private: void orderTraversal(TreeNode *root, size_t level, vector<vector<int> > & result) { if (root == NULL) return; if (level > result.size()) result.push_back(vector<int>()); result[level - 1].push_back(root->val); orderTraversal(root->left, level + 1, result); orderTraversal(root->right, level + 1, result); } }; ~~~ **四. 小結** 解法同Binary Tree Level Order Traversal，同樣有多種解法，還需認真研究。