A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

The easiest way to reverse a number is to convert the number to a string then to a character array and call reserve

F# 

let rev (s : string) = new string(s.ToCharArray() |> Array.rev)

C# 

n.ToString().ToCharArray().Reverse();

But instead I decided to use the math to reverse the number.

F# 

let toLargest l = List.fold1_left max l

 

let reverse n =

  let rec reverseRecursive x res =

    if x = 0 then res

    else reverseRecursive (x/10) (res*10 + (x%10))

 

  reverseRecursive n 0

 

let isPalindrome n =

  n = reverse n

  

let largestPalindrome =  

  [ for i in 100..999

    for j in 100..999

      when isPalindrome(i*j) -> i*j ] |> toLargest 

 

printf "%i"  largestPalindrome

C# 

static public class Numbers

{

    static private Int64 reverseRecursive(Int64 x, Int64 res)

    {

        if (x == 0)

            return res;

        else

            return reverseRecursive(x / 10, (res * 10 + (x % 10)));

    }

 

    static public Int64 Reverse(Int64 i)

    {

        return reverseRecursive(i, 0);

    }

 

    static public bool isPalindrome(this Int64 i)

    {

        return i == Reverse(i);

    }

}

 

 

class Program

{

    static void Main(string[] args)

    {

        Int64 maxPalindrome = Enumerable.Range(100, 900).

            SelectMany(i => Enumerable.Range(i, 900 - (i - 100)).Select(j => i * j)).

            Where (p => ((Int64) p).isPalindrome()).Max();

 

        Console.WriteLine(maxPalindrome);

        Console.ReadLine();

    }

}
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