@Override
  public E peek() throws Exception {
    // TODO Auto-generated method stub
    if (list.size() > 0) {
      return list.getFirst();
    }

    return null;
  }
 @Override
 public void empty() {
   // TODO Auto-generated method stub
   while (list.getFirst() != null) list.removeFirst();
 }
 @Override
 public E pop() throws Exception {
   // TODO Auto-generated method stub
   return list.removeFirst();
 }
 @Override
 public int size() {
   // TODO Auto-generated method stub
   return list.size();
 }
 @Override
 public void push(E x) {
   // TODO Auto-generated method stub
   list.addFirst(x);
 }