// Implements a stack of integers.
class IntStack {
    // Stack node type.
    private class StackNode {
	StackNode(int d, StackNode n) {
	    data = d;
	    next = n;
	}
	public int data;
	public StackNode next;
    }
    
    // Head of the stack
    private StackNode head = null;

    // Push onto the stack.
    public void push(int i) {
	head = new StackNode(i, head);
    }

    // Remove from the stack.
    public int pop() {
	if (head != null) {
            // Get the head and remove it.
            int retval = head.data;
	    head = head.next;

            // Return the value.
            return retval;
	} else
	    return 0;
    }

    // Find out what's on the top.
    public int top() {
	if(head == null)
	    return 0;
	else
	    return head.data;
    }

    // So, are we empty yet?
    public boolean isEmpty() {
	return head == null;
    }

    // Remove all memory from the stack.
    public void clear() {
	head = null;
    }
}