using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TodoApplication
{


    public class LimitedSizeStack<T>
    {

        class StackItem
        {
            public StackItem(T item)
            {
                this.item = item;
            }
            public T item;
            public StackItem NextItem;
            public StackItem LastItem;
        }

        public LimitedSizeStack(int limit)
        {
            stackLimit = limit;
        }

        int stackLimit;
        StackItem head;
        StackItem tail;
        int stackCount;

        public void Push(T item)
        {
            StackItem stackItem = new StackItem(item);
            if (head == null)
            {
                head = tail = stackItem;
                stackCount++;
            }
            else
            {
                tail.NextItem = stackItem;
                stackItem.LastItem = tail;
                tail = stackItem;
                stackCount++;
            }
            if (stackCount > stackLimit)
            {
                head = head.NextItem;
                head.LastItem = null;
                stackCount--;
            }
        }

        public T Pop()
        {
            if (head == null) throw new InvalidOperationException();
            var result = tail.item;
            tail = tail.LastItem;
            tail.NextItem = null;
            stackCount--;
            return result;
        }

        public int Count
        {
            get
            {
                return stackCount;
            }
        }


    }


}