def test_sorted():
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
assert mergesort(arr) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
def test_random():
arr = [3, 9, 11, 16, 14, 12, 15, 8, 10, 2, 5, 6, 13, 7, 4, 17, 1]
mergesort(arr)
assert arr == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]
def test_empty():
arr = []
assert mergesort(arr) == []
def test_one():
arr = [5]
assert mergesort(arr) == [5]
def test_backwards():
arr = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
assert mergesort(arr) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]