class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        # write code here
        if root is None:
            return 0
        if root.left is None and root.right is None:
            return 1
        l = self.maxDepth(root.left)
        r = self.maxDepth(root.right)
        return l+1 if l>r else r+1