using System;

using System.Collections.Generic;

​

​

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 patternLength) : base(patternLength){

    }

    public override void PrintNormal() {

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

    }

    public override void PrintVariant() {

        Console.WriteLine("*");

        Console.WriteLine("O   O");

    }

    public override void PrintSpecial() {

    }

}

​

​

public class Program

{

    public static void Main()

    {

        RiverPattern river = new RiverPattern(10);

        river.Print();

    }

}

​