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