interface Collection<A> {
  public void add(A x);
  public Iterator iterator();
}

interface Iterator<A> {
  public A next();
  public boolean hasNext();
}

class NoSuchElementException extends RuntimeException {}

class LinkedList<A> implements Collection<A> {
  protected class Node {
    A elt;
    Node next = null;
    Node(A elt){ this.elt = elt; }
  }

  protected Node head = null, tail = null;
  public LinkedList(){}
  public void add(A elt){
    if(head==null){ head = new Node(elt); tail = head; }
    else { tail.next = new Node(elt); tail = tail.next; }
  }

  public Iterator<A> iterator(){
    return new Iterator<A>(){
      protected Node prt = head;
      public boolean hasNext(){ return ptr!=null; }
      public A next(){
        if(ptr!=null){
          A elt=ptr.elt; ptr=ptr.next; return elt;
        } else
          throw new NoSuchElementException();
      }
    };
  }
}