public void AddLastTest()
        {
            var list = new DoubleLinkedListImpl <int>();

            list.AddLast(1);
            Assert.AreEqual(list.FirstOrDefault(), 1);
            Assert.IsNotNull(list.First);
        }
        public void ReverseTest()
        {
            var list = new DoubleLinkedListImpl <int>();

            list.AddLast(1);
            list.AddLast(2);
            list.AddLast(3);
            string s = "";

            foreach (var item in list)
            {
                s += $" {item.ToString()}";
            }
            Assert.AreEqual(s, " 1 2 3");
            list.Reverse();

            s = "";
            foreach (var item in list)
            {
                s += $" {item.ToString()}";
            }
            Assert.AreEqual(s, " 3 2 1");
        }