using System;
using System.Collections.Generic;

namespace Clones
{
	public class Clon
	{
		public List<string> izucheniecomand = new List<string>();
		public List<string> otcati = new List<string>();
		public bool cloneli = false;
	}
	public class CloneVersionSystem : ICloneVersionSystem
	{
		public Dictionary<string, Clon> dictionary = new Dictionary<string, Clon>() { { "1", new Clon() } };

		public string Execute(string query)
		{
			var array = query.Split(' ');
			if (!(dictionary.ContainsKey(array[1])))
				return null;
			switch (array[0])
			{
				case "learn":
					if (dictionary[array[1]].izucheniecomand.Contains(array[2])) break;
					dictionary[array[1]].izucheniecomand.Add(array[2]);
					break;

				case "rollback":
					if (dictionary[array[1]].izucheniecomand.Count == 0) break;
					dictionary[array[1]].otcati.Add((dictionary[array[1]].izucheniecomand[dictionary[array[1]].izucheniecomand.Count - 1]));
					dictionary[array[1]].izucheniecomand.RemoveAt(dictionary[array[1]].izucheniecomand.Count - 1);
					break;

				case "clone":
					if (dictionary[array[1]].cloneli == true) break;
					dictionary.Add((int.Parse(array[1]) + 1).ToString(), new Clon());
					dictionary[(int.Parse(array[1]) + 1).ToString()].izucheniecomand = new List<string>(dictionary[array[1]].izucheniecomand);
					dictionary[(int.Parse(array[1]) + 1).ToString()].otcati = new List<string>(dictionary[array[1]].otcati);
					dictionary[array[1]].cloneli = true;
					break;

				case "check":
					if (dictionary[array[1]].izucheniecomand.Count == 0) return "basic";
					else return dictionary[array[1]].izucheniecomand[dictionary[array[1]].izucheniecomand.Count - 1];
			}

			return null;
		}
	}
}