一. 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
二. 题目分析
由于使用了vector，这一题只需Binary Tree Level Order Traversal的基础上加一句reverse(result.begin(), result.end()) 即可。印象中编程之美上有这题。
三. 示例代码
;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. 小结
解法同Binary Tree Level Order Traversal，同样有多种解法，，还需认真研究。
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