// Write a function int Reverse(int n) that returns the “decimal digit reversal”

// of the integer number n > 0. 

// For instance, Reverse(123) should return 321.

// Only integer arithmetic is allowed, no string operations.

using System;

using System.Diagnostics;

​

public class Program

{

    static public int Reverse(int needToReverse)

    {

        Trace.Assert(needToReverse > 0);

        return Reverse(0, needToReverse);

    }

    // Returns integer number containing the digits of alreadyReversed in normal order

    // and then digits of needToReverse in reversed order.

    // Both parameters can be 0, no digits is added by such parameter.

    // Completion: number of digits in needToReverse decreases in each nested call.

    static public int Reverse(int alreadyReversed, int needToReverse)

    {

        if(0==needToReverse)

            return alreadyReversed;

        return Reverse(10 * alreadyReversed + needToReverse % 10, needToReverse / 10);

    }

    static public void Main()

    {

        foreach(int n in new int[]{ 1, 12, 123, 2005, 333, 20, 2018})

            Console.WriteLine("{0} -> {1}", n, Reverse(n));

    }

}