Simple Text Editor

            int n = int.Parse(Console.ReadLine());

            var builder = new StringBuilder();
            var stack = new Stack<string>();
            stack.Push(builder.ToString());

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

                switch (command)
                {
                    case 1:
                        builder.Append(input[1]);
                        stack.Push(builder.ToString());
                        break;
                    case 2:
                        int number = int.Parse(input[1]);
                        builder.Remove(builder.Length - number, number);
                        stack.Push(builder.ToString());
                        break;
                    case 3:
                        int index = int.Parse(input[1]);
                        Console.WriteLine(builder[index - 1]);
                        break;
                    case 4:
                        stack.Pop();
                        builder = new StringBuilder();
                        builder.Append(stack.Peek());
                        break;
                }
            }