using System;

public class Program

{

    public static int GetMaxNumber(int[] input, int idx = 0)

    {

        if (idx < input.Length - 1)

            return Math.Max(input[idx], GetMaxNumber(input, idx + 1));

        else

            return input[idx];

    }

    public static void Main()

    {

        int[] input = {1,5,12,3,33,61,42,9,23};

        Console.WriteLine($"{arrayToString(input)} => max number: {GetMaxNumber(input)}");

    }

​

​

    // helpers

    private static string arrayToString(int[] array)

    {

        string s = "{";

        if (array.Length > 0) s += string.Join(',', array);

        s += "}";

        return s;

    }

}