def test_empty_tree(self):
tree = BinaryTree()
assert tree.root is None
assert tree.is_empty() is True
assert tree.size == 0
assert tree.levels == 0
assert tree.__repr__() == "BinaryTree(serialize='')"
assert tree.serialize() == ''
assert tree.level_order() == []
assert tree.pre_order() == []
assert tree.in_order() == []
assert tree.post_order() == []
assert tree == BinaryTree()
