using System;
using System.Collections.Generic;

namespace Clones
{
	public class StackItem
	{
		public string Value { get; set; }
		public StackItem Next { get; set; }
	}

	public class Stack
	{
		public StackItem tail;
		public void Push(string value)
		{
			if (tail == null)
				tail = new StackItem { Value = value, Next = null };
			else
			{
				var item = new StackItem { Value = value, Next = tail };
				tail = item;
			}
		}

		public string Depush()
		{
			if (tail == null) throw new InvalidOperationException();
			var result = tail.Value;
			tail = tail.Next;
			return result.ToString();
		}

		internal Stack GetCopy()
		{
			return new Stack { tail = tail };
		}
	}

	public class Clone
	{
		public Stack learnedProgramm = new Stack();
		public Stack unlearnedProgramm = new Stack();

		public Clone()
		{
			if (learnedProgramm.tail == null)
				learnedProgramm.Push("basic");
		}

		public void Learn(string programm)
		{
			learnedProgramm.Push(programm);
		}
		public void Rollback()
		{
			var lastProgramm = learnedProgramm.Depush();
			unlearnedProgramm.Push(lastProgramm);
		}

		public Clone GetCLone()
		{
			return new Clone { learnedProgramm = learnedProgramm.GetCopy(), unlearnedProgramm = unlearnedProgramm };
		}

		public string Chek()
		{
			return learnedProgramm.tail.Value;
		}

		public void Relearn()
		{
			learnedProgramm.Push(unlearnedProgramm.Depush());
		}
	}

	public class CloneVersionSystem : ICloneVersionSystem
	{
		public List<Clone> clones = new List<Clone>();
		public CloneVersionSystem()
			=> clones.Add(new Clone());
		public string Execute(string query)
		{
			var instruction = query.Split();
			switch (instruction[0])
			{
				case "learn": clones[int.Parse(instruction[1]) - 1].Learn(instruction[2]); break;
				case "rollback": clones[int.Parse(instruction[1]) - 1].Rollback(); break;
				case "check": return clones[int.Parse(instruction[1]) - 1].learnedProgramm.tail.Value;
				case "relearn": clones[int.Parse(instruction[1]) - 1].Relearn(); break;
				case "clone": clones.Add(clones[int.Parse(instruction[1]) - 1].GetCLone()); break;
			}
			return null;
		}
	}
}