public class intStackList extends intStack {
    static class Node {
	int val;
	Node next;
	Node (int v, Node t) { val= v; next= t; }
    }

    private Node head;

    intStackList (int n) { head = new Node (0, null); }

    public boolean empty () { return head.next == null; }

    public void push (int i) {
	Node t = new Node (i, head.next);
	head.next = t;
    }

    public int pop () {
	int i = head.next.val;
	head.next = head.next.next;
	return i;
    }
}