//this method test PeekHead() function
        public void PushBack123_1returned()
        {
            int expected = 1;
            int list_buffer;

            simple_ds.Structures.DoubleLinkedList <int> list = new simple_ds.Structures.DoubleLinkedList <int>();

            for (var number = 1; number <= 3; ++number)
            {
                list.PushBack(number);
            }

            list_buffer = list.PeekHead();

            Assert.AreEqual(expected, list_buffer);
        }