using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var engine = new RedisEngine();

        //engine.SET("ex", 10);

        //engine.GET("ex");

        //engine.UNSET("ex");

        //engine.GET("ex");

        //engine.SET("a", 10);

        //engine.SET("b", 10);

        //engine.NUMEQUALTO(10);

        //engine.NUMEQUALTO(20);

        //engine.SET("b", 30);

        //engine.NUMEQUALTO(10);

        //engine.BEGIN();

        //engine.SET("a", 10);

        //engine.GET("a");

        //engine.BEGIN();

        //engine.SET("a", 20);

        //engine.GET("a");

        //engine.ROLLBACK();

        //engine.GET("a");

        //engine.ROLLBACK();

        //engine.GET("a");

        //engine.BEGIN();

        //engine.SET("a", 30);

        //engine.BEGIN();

        //engine.SET("a", 40);

        //engine.COMMIT();

        //engine.GET("a");

        //engine.ROLLBACK();

        engine.SET("a", 50);

        engine.BEGIN();

        engine.GET("a");

        engine.SET("a", 60);

        engine.BEGIN();

        engine.UNSET("a");

        engine.GET("a");

        engine.ROLLBACK();

        engine.GET("a");

        engine.COMMIT();

        engine.GET("a");

        //engine.ROLLBACK();

        //engine.GET("a");

        //engine.ROLLBACK();

        //engine.GET("a");

        Console.WriteLine("END");

    }

    public class RedisEngine

    {

        public Dictionary<string, int> entries = new Dictionary<string, int>();

        public readonly Stack<Dictionary<string, int>> transactions = new Stack<Dictionary<string, int>>();

        public RedisEngine()

        {

        }

        public bool SET(string key, int value)

        {

            if (!this.entries.ContainsKey(key))

            {

                this.entries.Add(key, value);

                return true;

            }

            else

            {

                this.entries[key] = value;

            }

            return false;

        }

        public int? GET(string key)

        {

            if (this.entries.ContainsKey(key))

            {

                Console.WriteLine(this.entries[key]);

                return this.entries[key];

            }

            Console.WriteLine("NULL");

            return null;

        }

        public bool UNSET(string key)

        {

            if (this.entries.ContainsKey(key))

            {

                this.entries.Remove(key);

                return true;

            }

            return false;

        }

        public int NUMEQUALTO(int value)

        {

            var count = this.entries.Count(x => x.Value == value);

            Console.WriteLine(count);

            return count;

        }

        public void BEGIN()

        {

            transactions.Push(new Dictionary<string, int>(entries));

        }

        public void ROLLBACK()

        {

            if (transactions.Count > 0)

            {

                entries = transactions.Pop();

            }

            else

            {

                Console.WriteLine("NO TRANSACTION");

            }

        }

        public void COMMIT()

        {

            transactions.Clear();

        }

    }

}