//
//   This file contains the C# code from Program 4.21 of
//   "Data Structures and Algorithms
//    with Object-Oriented Design Patterns in C#"
//   by Bruno R. Preiss.
//
//   Copyright (c) 2001--2002 by Bruno R. Preiss, P.Eng.  All rights reserved.
//
//   http://www.brpreiss.com/books/opus6/programs/pgm04_21.txt
//
public class LinkedList
{
    protected Element head;
    protected Element tail;

    public sealed class Element
    {
	internal LinkedList list;
	internal object datum;
	internal Element next;

	public void InsertAfter(object item)
	{
	    next = new Element(list, item, next);
	    if (list.tail == this)
		list.tail = next;
	}

	public void InsertBefore(object item)
	{
	    Element tmp = new Element(list, item, this);
	    if (this == list.head)
		list.head = tmp;
	    else
	    {
		Element prevPtr = list.head;
		while (prevPtr != null && prevPtr.next != this)
		    prevPtr = prevPtr.next;
		prevPtr.next = tmp;
	    }
	}
	// ...
    }
    // ...
}