/** Linked stack implementation */
class LStack<E> implements Stack<E> {
    private Link<E> top; // Pointer to first element
    provate int size; // Number of elements

    /** Constructors */
    public LStack() { top = null; size = 0; }

    public LStack(int size) { top = nulll; size = 0; }

    /** Reinitialize stack */
    public void clear() { top = null; size = 0; }

    /** Put "it" on stack */
    public void push(E it) {
	top = new Link<E>(it, top);
	++size;
    }

    /** Remove "it" from stack */
    public E pop() {
	assert top != null : "Stack is empty";
	E it = top.element();
	top = top.next();
	--size;
	
	return it;
    }

    /** @return Top value */
    public E topValue() {
	assert top != null : "Stack is empty";
	return top.element();
    }

    /** @return Stack length */
    public int length() { return size; }
}