def test_custom_operations():
new_list = SinglyLinkedList()
new_list.addAtIndex(0, 10)
new_list.addAtIndex(0, 20)
new_list.addAtIndex(1, 30)
assert new_list.head.val == 20
assert new_list.tail.val == 10
assert new_list.length == 3
assert new_list.get(0) == 20
def test_linked_list_insert_middle_index():
new_list = SinglyLinkedList()
new_list.addAtHead(4)
new_list.addAtTail(6)
new_list.addAtIndex(1, 5)
assert new_list.head.val == 4
assert new_list.tail.val == 6
assert new_list.head.next.val == 5
assert new_list.head.next.next.val == 6
assert new_list.length == 3
def test_linked_list_delete_middle():
new_list = SinglyLinkedList()
new_list.addAtHead(7)
new_list.addAtHead(2)
new_list.addAtHead(1)
new_list.addAtIndex(3, 0)
new_list.deleteAtIndex(2)
new_list.addAtHead(6)
new_list.addAtTail(4)
assert new_list.head.val == 6
assert new_list.tail.val == 4
assert new_list.length == 4
assert new_list.get(4) == 4
def test_linked_list_insert_invalid_index():
new_list = SinglyLinkedList()
new_list.addAtHead(5)
new_list.addAtTail(6)
assert new_list.addAtIndex(4, 10) == None
assert new_list.length == 2
