class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:

        if not root:
            return 0

        return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))

class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:

        if root is None:
            return 0

        left_count = self.maxDepth(root.left)
        right_count = self.maxDepth(root.right)

        return 1 + max(left_count, right_count)