public class StackSoluce<E> {
	StackNode<E> head;
	StackSoluce() { head = null; }
	
	/* question 3.a */
	public void push(E element) { 
		head = new StackNode<E>(head, element);
	}
	
	/* question 3.b */
	public E pop() {
		if(head == null)
			return null;
		else {
			E res = head.element;
			head = head.next;
			return res;
		}
	}
}