using System;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var test = new AlphabetEnumerable

        {

            "z", "Bravo", "alpha", "D", "u"

            //"a", "b", "c", "z"

        };

        Console.WriteLine("Round 1");

        foreach (var letter in test)

            Console.WriteLine(letter);

        Console.WriteLine("=============");

        Console.WriteLine("Round 2");

        foreach (var letter in test)

            Console.WriteLine(letter);

        Console.WriteLine("=============");

        var test2 = new AlphabetEnumerable();

        Console.WriteLine("Empty");

        foreach (var letter in test2)

            Console.WriteLine(letter);

    }

    public class AlphabetEnumerable : LinkedList<string>, IEnumerable

    {

        public void Add(string val)

        {   

            var currentNode = base.First;

            while (currentNode != null && currentNode.Value.CompareTo(val) < 0)

                currentNode = currentNode.Next;

            if (currentNode == null)

                base.AddLast(val);

            else

                base.AddBefore(currentNode, val);

        }

        new public IEnumerator GetEnumerator()

        {

            return new AlphabetEnumerator(this.First);

        }

    }

    public class AlphabetEnumerator : IEnumerator

    {

        private LinkedListNode<string> _firstNode;

        private LinkedListNode<string> _currentNode;

        public Object Current {get { return _currentNode == null? null : _currentNode.Value; } }

        public AlphabetEnumerator(LinkedListNode<string> firstNode)

        {

            _firstNode = firstNode;

        }

        public bool MoveNext()

        {

            if (_currentNode == null)

                _currentNode = _firstNode;

            else

                _currentNode = _currentNode.Next;

            return _currentNode != null;

        }

        public void Reset()

        {

            _currentNode = null;

        }

    }

}