55 - I. 二叉树的深度

题目链接：https://leetcode-cn.com/problems/er-cha-shu-de-shen-du-lcof/
难度：简单

描述：
输入一棵二叉树的根节点，求该树的深度。从根节点到叶节点依次经过的节点（含根、叶节点）形成树的一条路径，最长路径的长度为树的深度。

题解

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        def recursion(root, depth):
            if root is None:
                return depth-1
            return max(recursion(root.left, depth+1), recursion(root.right, depth+1))
        return recursion(root, 1)