def test_get_next__successor_complex():
A = BinaryNode(1)
C = BinaryNode(3)
B = BinaryNode(2, left=A, right=C)
D = BinaryNode(4, left=B, right=None)
assert get_next(A).value == 2
assert get_next(B).value == 3
assert get_next(C).value == 4
assert get_next(D) is None
def test_get_next__no_successor_self_is_at_root():
assert get_next(BinaryNode(1)) is None
def test_get_next__nothing():
assert get_next(None) is None
def test_get_next__parent_is_successor():
node = BinaryNode(2, left=BinaryNode(1))
assert get_next(node.left).value == 2
def test_get_next__simple_successor():
node = BinaryNode(1, right=BinaryNode(2))
assert get_next(node).value == 2