def test_validate():
root = Node(1, Node(2))
assert not root.validate()
root = Node(1, Node(0))
assert root.validate()
root = Node(1, None, Node(0))
assert not root.validate()
root = Node(1, None, Node(2))
assert root.validate()
root = Node(1, Node(0), Node(2))
assert root.validate()
