public void AddTwoNodesHead()
{
circularDoublyLinkedList.AddFirst(1);
circularDoublyLinkedList.AddFirst(2);
Assert.AreEqual(circularDoublyLinkedList.GetFirst(), 2, "invalid head element");
Assert.AreEqual(circularDoublyLinkedList.GetLast(), 1, "invalid tail element");
}
