import java.util.*;

// Simple Java array-based stack template class.
public class Stack<T> 
{
    private Vector<T> data; // Should be T[] data;, but Java forbids it.
    private int top_elt;

    // Create an empty stack of a given size.
    public Stack(int size) 
    { 
	data = new Vector<T>(size);
	data.setSize(size);
	top_elt = -1;
    }

    // Add an element to the stack.  Note that you must send a pointer,
    // and the stack actually stores the pointer.
    public void push(T e)
    {
	data.set(++top_elt, e);
    }

    // Pop the stack and return the top item pointer.  If the stack is
    // empty, return the null pointer.
    public T pop() 
    {
	if(empty()) return null;
	else return data.get(top_elt--);
    }

    // Return the top item pointer.  If none, return the null pointer.
    public T top() 
    {
	if(empty()) return null;
	return data.get(top_elt);
    }

    // Tell if the stack is empty.
    public Boolean empty() { return top_elt == -1; }
}