using System;

namespace MyCollections
{
    public class Stack //why public?
    {
        Entry top;

        public void Push(object data)
        {
            top = new Entry(top, data);
        }

        public object Pop()
        {
            if (top == null) throw new InvalidOperationException();
            object result = top.data;
            top = top.next;
            return result;
        }

        class Entry
        {
            public Entry next;
            public object data;

            public Entry(Entry n, object d)
            {
                next = n;
                data = d;
            }
        }
    }


}