def serialize(root_node) -> str:
	"""
	Serializes the tree into a string of the form Node [LeftTree] [RightTree]
	:param root_node: The root of the tree
	:return: A string representing the serialized version of the tree
	"""
	if root_node is None:
		return ""
	elif root_node.left is None and root_node.right is None:
		return f"{root_node.val} [] []"
	else:
		return (
			f"{root_node.val} "
			f"[{serialize(root_node.left)}] "
			f"[{serialize(root_node.right)}]"
		)