text: """
  
  link: https://leetcode.com/problems/maximum-depth-of-binary-tree
  
  problem: 求树的深度
  
  solution: 基操。
  
  """
  
  # 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:
          if not root:
              return 0
          return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1