class LinkedNode {
  private Object o;
  private LinkedNode next;
  
  public LinkedNode(LinkedNode nxt, Object e) {
    next = nxt;
    o = e;
  }
  
  public LinkedNode(Object e) {
    this(null, e);
  }
  
  public Object getElement() { return o; }
  public LinkedNode getNext() { return next; }
  
  public void setElement(Object e) { o = e; }
  public void setNext(LinkedNode node) { next = node; }
}

public class LinkedStack implements Stack {
  int size;
  LinkedNode head;

  public LinkedStack() {
    size=0;
    head = null;
  }
  
  public int size() {
    return size;
  }
  
  public void push(Object o) {
    // the following is OK even if head == null.
    head = new LinkedNode(head, o);
    size++;
  }
  
  public Object pop() {
    LinkedNode pN = head;
    head = head.getNext();
    size--;
    return pN.getElement();
  }
  
  public Object top() {
    return head;
  }
  
  public String toString() {
    String str = "@@@@@@@@ - Stack - @@@@@@@@\n";
    LinkedNode curr = head;
    while(curr != null)
      str += curr.getElement().toString();
    return str + "@@@@@@@@@@@@@@@@@@@@@@@@@@@\n";
  }
}