using System;
using System.Collections.Generic;

namespace Clones
{
    public class CloneVersionSystem : ICloneVersionSystem
    {
        private List<Clone> clones;
        public string Execute(string query)
        {
            var allCommands = query.Split();
            var commandName = allCommands[0];
            var cloneId = int.Parse(allCommands[1]);
            var programId = allCommands.Length > 2 ? allCommands[2] : "";
            if (clones.Count < cloneId) return null;
            return RunCommand(commandName, clones[cloneId - 1], programId);
        }

        public CloneVersionSystem()
        {
            clones = new List<Clone>{new Clone()};
        }

        private string RunCommand(string commandName, Clone clone, string programId)
        {
            string result = null;
            switch (commandName)
            {
                case "learn":
                    LearnClone(clone, programId);
                    break;
                case "rollback":
                    RollbackClone(clone);
                    break;
                case "relearn":
                    RelearnClone(clone);
                    break;
                case "clone":
                    ToCloneClone(clone);
                    break;
                case "check":
                    result = CheckClone(clone);
                    break;
            }
            return result;
        }

        private void LearnClone(Clone clone, string programId)
        {
            clone.HistoryStack.Clear();
            clone.ProgramsStack.Push(int.Parse(programId));
        }

        private void RollbackClone(Clone clone)
        {
            clone.HistoryStack.Push(clone.ProgramsStack.Pop());
        }

        private void RelearnClone(Clone clone)
        {
            clone.ProgramsStack.Push(clone.HistoryStack.Pop());
        }

        private string CheckClone(Clone clone)
        {
            if (clone.ProgramsStack.IsEmpty()) return "basic";
            return clone.ProgramsStack.Peek()+"";
        }

        private void ToCloneClone(Clone clone)
        {
            var newClone = clone.CloneThisClone();
            clones.Add(newClone);
        }
    }

    public class Clone
    {
        public Stack ProgramsStack;
        public Stack HistoryStack;

        public Clone()
        {
            ProgramsStack = new Stack();
            HistoryStack = new Stack();
        }

        public Clone CloneThisClone()
        {
            var other = new Clone
            {
                ProgramsStack = new Stack(ProgramsStack),
                HistoryStack = new Stack(HistoryStack)
            };
            return other;
        }

        public class Stack
        {
            private StackItem last;

            public Stack()
            {
            }

            public Stack(Stack stack)
            {
                last = stack.last;
            }

            public void Push(int value)
            {
                last = new StackItem(value, last);
            }
       using System;
using System.Collections.Generic;

namespace Clones
{
    public class CloneVersionSystem : ICloneVersionSystem
    {
        private List<Clone> clones;
        public string Execute(string query)
        {
            var allCommands = query.Split();
            var commandName = allCommands[0];
            var cloneId = int.Parse(allCommands[1]);
            var programId = allCommands.Length > 2 ? allCommands[2] : "";
            if (clones.Count < cloneId) return null;
            return RunCommand(commandName, clones[cloneId - 1], programId);
        }

        public CloneVersionSystem()
        {
            clones = new List<Clone>{new Clone()};
        }

        private string RunCommand(string commandName, Clone clone, string programId)
        {
            string result = null;
            switch (commandName)
            {
                case "learn":
                    LearnClone(clone, programId);
                    break;
                case "rollback":
                    RollbackClone(clone);
                    break;
                case "relearn":
                    RelearnClone(clone);
                    break;
                case "clone":
                    ToCloneClone(clone);
                    break;
                case "check":
                    result = CheckClone(clone);
                    break;
            }
            return result;
        }

        private void LearnClone(Clone clone, string programId)
        {
            clone.HistoryStack.Clear();
            clone.ProgramsStack.Push(int.Parse(programId));
        }

        private void RollbackClone(Clone clone)
        {
            clone.HistoryStack.Push(clone.ProgramsStack.Pop());
        }

        private void RelearnClone(Clone clone)
        {
            clone.ProgramsStack.Push(clone.HistoryStack.Pop());
        }

        private string CheckClone(Clone clone)
        {
            if (clone.ProgramsStack.IsEmpty()) return "basic";
            return clone.ProgramsStack.Peek()+"";
        }

        private void ToCloneClone(Clone clone)
        {
            var newClone = clone.CloneThisClone();
            clones.Add(newClone);
        }
    }

    public class Clone
    {
        public Stack ProgramsStack;
        public Stack HistoryStack;

        public Clone()
        {
            ProgramsStack = new Stack();
            HistoryStack = new Stack();
        }

        public Clone CloneThisClone()
        {
            var other = new Clone
            {
                ProgramsStack = new Stack(ProgramsStack),
                HistoryStack = new Stack(HistoryStack)
            };
            return other;
        }

        public class Stack
        {
            private StackItem last;

            public Stack()
            {
            }

            public Stack(Stack stack)
            {
                last = stack.last;
            }

            public void Push(int value)
            {
                last = new StackItem(value, last);
            }

            public int Pop()
            {
                var value = last.Value;
                last = last.Previous;
                return value;
            }

            public void Clear()
            {
                last = null;
            }

            public int Peek()
            {
                return last.Value;
            }

            public bool IsEmpty()
            {
                return last == null;
            }
        }

        public class StackItem
        {
            public readonly int Value;
            public readonly StackItem Previous;

            public StackItem(int value, StackItem previous)
            {
                Value = value;
                Previous = previous;
            }
        }
    }
}
            public int Pop()
            {
                var value = last.Value;
                last = last.Previous;
                return value;
            }

            public void Clear()
            {
                last = null;
            }

            public int Peek()
            {
                return last.Value;
            }

            public bool IsEmpty()
            {
                return last == null;
            }
        }

        public class StackItem
        {
            public readonly int Value;
            public readonly StackItem Previous;

            public StackItem(int value, StackItem previous)
            {
                Value = value;
                Previous = previous;
            }
        }
    }
}