using System;

​

public abstract class PatternPrinter

{

    private int patternLength;

    public PatternPrinter(int patternLength)

    {

        this.patternLength = patternLength;

    }

​

    public void Print()

    {

        Random rnd = new Random();

        int specialIndex = rnd.Next(patternLength);

        for (int i = 0; i < patternLength; i++)

        {

            if (i == specialIndex)

                PrintSpecial();

            else

            {

                int roll = rnd.Next(2); // either 0 or 1

                if (roll == 0)

                    PrintNormal();

                else

                    PrintVariant();

            }

        }

    }

​

    public abstract void PrintNormal();

    public abstract void PrintVariant();

    public abstract void PrintSpecial();

}

​

​

public class RiverPattern : PatternPrinter{

    public RiverPattern(int length) : base(length){}

    public override void PrintNormal(){

        for (int i=0; i<4; i++)

            Console.WriteLine("***O   O***");

    }

    public override void PrintVariant(){

        Console.WriteLine("**O   O****");

        Console.WriteLine("*O   O*****");

        Console.WriteLine("**O   O****");

        Console.WriteLine("***O   O***");

    }

    public override void PrintSpecial(){

        Console.WriteLine("**O    O***");

        Console.WriteLine("*O      O**");

        Console.WriteLine("*O  OO   O*");

        Console.WriteLine("*O  O    O*");

        Console.WriteLine("**O     O**");

    }

}

​

​

public class Program

{

    public static void Main()

    {

        RiverPattern river = new RiverPattern(10);

        river.Print();

    }

}