//Title: Singly-Linked List Implementation
//Version:      SLinkedList.java,v 1.3 2002/11/26 10:47:53 pbj Exp
//Copyright:    Copyright (c) 2002
//Author:       Paul Jackson
//Organisation: School of Informatics, University of Edinburgh




public class SLinkedList {
  protected SLink firstLink = null;

  public SLinkedList() {}

  public boolean isEmpty() { return firstLink == null;}

  public void addFirst(Object o) {
    firstLink = new SLink(o, firstLink);
  }

  public Object getFirst() {
    if (isEmpty()) return null;
    else return firstLink.element;
  }

  public Object removeFirst() {
    if (isEmpty()) return null;
    Object result = firstLink.element;
    firstLink = firstLink.nextLink;
    return result;
  }

  public boolean contains (Object o) {
    for (SLink i = firstLink; i != null; i = i.nextLink) {
      if (i.element.equals(o)) return true;
    }
    return false;
  }
  // A recursive version of contains().

  private boolean recContainsAux (Object o, SLink s) {
    if (s == null) return false;
    else if (s.element.equals(o)) return true;
    else return recContainsAux(o, s.nextLink);
  }

  public boolean recContains(Object o) {
    return recContainsAux(o, firstLink);
  }

  public Iterator iterator() {
    return new Iterator(firstLink);
  }
}