class Stack
{
 private Cell llist;

 public Object top()
 {
  return llist.first;
 }

 public void pop()
 {
  llist = llist.next;
 }

 public void push(Object obj)
 {
  llist = new Cell(obj,llist);
 }

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

 private static class Cell
 {
  Object first;
  Cell next;

  Cell(Object f,Cell n)
  {
   first=f;
   next=n;
  }
 }
}