104. Maximum Depth of Binary Tree-二叉树的最大深度

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:
Input: root = [3,9,20,null,null,15,7]

Output: 3

Example 2:
Input: root = [1,null,2]
Output: 2

Example 3:
Input: root = []
Output: 0

Example 4:
Input: root = [0]
Output: 1

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• -100 <= Node.val <= 100

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root == None:
            return 0
        else:
            return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))