text: # 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 not root:return 0
          return max(self.maxDepth(root.left),self.maxDepth(root.right))+1