public class StackLL<S> implements StackIntf<S> {
    private SingleLinkedList<S> underlyingLL;

    public StackLL() {
        underlyingLL = new SingleLinkedList<S>();
    }
    @Override
    public boolean isEmpty() {
        return underlyingLL.isEmpty();
    }

    @Override
    public S push(S e) {
        underlyingLL.addFirst(e);
        return e;
    }

    @Override
    public S peek() {
        return underlyingLL.first();
    }

    @Override
    public S pop() {
        return underlyingLL.removeFirst();
    }

       @Override
    public int size() {
        return underlyingLL.size();
    }

    @Override
    public void clear() {
        underlyingLL = new SingleLinkedList<S>();
    }
    
}