Binary Tree Level Order Traversal II

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本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的C++实现示例。该算法首先计算二叉树的高度，然后通过递归方式收集每个层级的节点值。

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},

return its bottom-up level order traversal as:

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

/**
 * Definition for binary tree
 * 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) {
        int level=getHeight(root);
        vector<vector<int> > res(level);
       
        levelOrder(res,root,level-1);
        return res;
    }
    void levelOrder(vector<vector<int> > &res,TreeNode *root,int level){
        if(root==NULL) return;
        res[level].push_back(root->val);
        if(root->left) levelOrder(res,root->left,level-1);
        if(root->right) levelOrder(res,root->right,level-1);
    }
    int getHeight(TreeNode * root){
        if(root==NULL) return 0;
        int left=getHeight(root->left);
        int right=getHeight(root->right);
        return left>right?(left+1):(right+1);
    }
};