public void InsertAtHeadTest1()
        {
            DoublyEndedList doublyEndedList = new DoublyEndedList();

            doublyEndedList.InsertAtHead(1);

            Assert.Equal(expected: "1,", actual: doublyEndedList.ToString());
        }
        public void InsertAtTailTest2()
        {
            DoublyEndedList doublyEndedList = new DoublyEndedList();

            doublyEndedList.InsertAtTail(6);

            Assert.Equal(expected: "6,", actual: doublyEndedList.ToString());
        }