难度：简单
一、题目描述：
二、解题分析：
1、剑指解析
I、自顶向下
II、自底向上
2、代码实现
I、自顶向下
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
# Compute the tree's height via recursion
def height(
