# 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: Optional[TreeNode]) -> int:
        ans = 0
        if not root: return 0
        def dfs(root, d):
            nonlocal ans
            if not root:
                ans = max(ans, d)
                return
            d += 1
            dfs(root.left, d)
            dfs(root.right, d)
        dfs(root, 0)
        return ans

class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        if root is None:
            return 0
        l_depth = self.maxDepth(root.left)
        r_depth = self.maxDepth(root.right)
        return max(l_depth, r_depth) + 1

作者：灵茶山艾府
链接：https://leetcode.cn/problems/maximum-depth-of-binary-tree/solutions/2010612/kan-wan-zhe-ge-shi-pin-rang-ni-dui-di-gu-44uz/
来源：力扣（LeetCode）
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