def count_nodes(self):
        """
        Count the number of Nodes in this subtree.
        :rtype: int
        """
        count = 1
        if self.left_node:
            count += self.left_node.count_nodes()
        if self.right_node:
            count += self.right_node.count_nodes()
        return count
Count the number of Nodes in this subtree.
        :rtype: int