SimpleTextEditor

using System;
using System.Collections.Generic;
using System.Text;

namespace _09.SimpleTextEditor
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            StringBuilder text = new StringBuilder();

            Stack<string> versions = new Stack<string>();

            for (int i = 0; i < n; i++)
            {
                string[] input = Console.ReadLine().Split();

                string command = input[0];

                if (command != "4")
                {
                    string action = input[1];

                    if (command == "1")
                    {
                        versions.Push(text.ToString());
                        text.Append(action);
                    }
                    else if (command == "2")
                    {
                        versions.Push(text.ToString());
                        text.Remove(text.Length - int.Parse(action), int.Parse(action));
                    }
                    else if (command == "3")
                    {
                        Console.WriteLine(text[int.Parse(action) - 1]);
                    }

                }
                else
                {
                    text = new StringBuilder();
                    text.Append(versions.Pop());
                }


            }
        }
    }
}