class Solution(object):
    def maxDepth(self, root):
        """ :type root: TreeNode :rtype: int """
        if root is None:
            return 0
        if root.left is None and root.right is None:
            return 1
        elif root.left is None:
            return self.maxDepth(root.right) + 1
        elif root.right is None:
            return self.maxDepth(root.left) + 1 
        else:
            depth_left = self.maxDepth(root.left)
            depth_right = self.maxDepth(root.right)
            if depth_left > depth_right:
                return depth_left + 1
            else:
                return depth_right + 1

Runtime: 64 ms