class Node {

	private Object o;

	private Node next;

	public Node(Object o) {
		this.o = o;
	}
}


class Stack {

	private Node first;

	private Node last;

	public void push(Object o) {

		if (first == null) {
			first = new Node(o);
			last = first;
			return;
		} else {
			last.setNext(new Node(o));
			last = last.getNext();

		}

	}

	public Object pop() {
		if (first == null) {
			return null;
		}

		if (first == last) {
			Object temp = first.getObject();
			first = null;
			return temp;
		}
		Object temp = last.getObject();
		setNewLast();

		return temp;

	}
}