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

namespace TodoApplication
{
    public class LimitedSizeStack<T>
    {
        LimitedSizeStackItem<T> head;
        LimitedSizeStackItem<T> tail;
        int count;
        int stackLimit;

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

        public void Push(T item)
        {
            if (!isLimitZero())
            {
                if (head == null)
                {
                    tail = head = new LimitedSizeStackItem<T> { Item = item, Next = null, Previous = null };
                    count++;
                }
                else
                {
                    if (isStackFull())
                    {
                        head = head.Next;
                        head.Previous = null;
                    }
                    else
                        count++;
                    var stackItem = new LimitedSizeStackItem<T> { Item = item, Next = null, Previous = tail };
                    tail.Next = stackItem;
                    tail = stackItem;
                }
            }
        }

        public T Pop()
        {
            if (!isLimitZero() && count != 0)
            {
                var result = tail.Item;
                count--;
                if (tail != head)
                    tail = tail.Previous;
                if (head == null)
                    tail = null;
                return result;
            }
            return default;
        }

        public int Count
        {
            get
            {
                return count;
            }
        }

        private bool isStackFull()
        {
            return count == stackLimit;
        }

        private bool isLimitZero()
        {
            return stackLimit == 0;
        }
    }

    public class LimitedSizeStackItem<T>
    {
        public T Item { get; set; }
        public LimitedSizeStackItem<T> Next { get; set; }
        public LimitedSizeStackItem<T> Previous { get; set; }
    }
}