MyStack.java

public class MyStack {

    private Node top;

    public MyStack() {
        super();
        top = new Node(null, null);
    }

    public int size() {
        return top.getHeight();
    }

    public void push(Object element) {
        this.top = new Node(element, top);
    }

    public Object pop() {
        Object payload = top.getPayload();
        top = top.getPrevious();
        return payload;
    }

    private class Node {
        private Object payload;
        private int height;
        private Node previous;

        Node(Object payload, Node oldTop) {
            this.payload = payload;
            this.previous = oldTop;
            height = (oldTop == null) ? 0 : oldTop.height+1;
        }

        Object getPayload() {return payload;}
        int getHeight() {return height;}
        Node getPrevious() {return previous;}
    }
}