def maxDepth(self, root: TreeNode) -> int:
    if (root is None):
        return 0
    return (1 + max(self.maxDepth(root.left), self.maxDepth(root.right)))
def maxDepth(self, root: TreeNode) -> int:
    if (root is None):
        return 0
    return (1 + max(self.maxDepth(root.left), self.maxDepth(root.right)))