using System;

using System.Collections.Generic;

​

public class EditorState 

{

    private string content;

​

    public EditorState(string content) {

        this.content = content;

    }

​

    public string Content {

        get 

        {

            return content;

        }

    }

}

​

public class Editor 

{

    public string Content {get; set; }

​

    public EditorState createState() {

        return new EditorState(Content);

    }

​

    public void restore(EditorState state) {

        Content = state.Content;

    }

}

​

public class History 

{

    private List<EditorState> states = new List<EditorState>();

​

    public void push(EditorState state) {

        states.Add(state);

    }

​

    public EditorState pop() {

        var lastIndex = states.Count - 1;

        var lastState = states[lastIndex];

        states.Remove(lastState);

​

        return lastState;

    }

}

public class Program

{

    public static void Main()

    {

        var editor = new Editor();

        var history = new History();

​

        editor.Content = "hello";

        history.push(editor.createState());

​

        editor.Content = "hello world";

        history.push(editor.createState());

​

        editor.Content = "hello world design patterns";

        editor.restore(history.pop());

        Console.WriteLine(editor.Content);

    }

}