using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(GetLongestPalindrome("daadrefer"));

        Console.WriteLine(GetLongestPalindrome("abcd"));

    }

    public static string GetLongestPalindrome(string input) 

    {

        var inputChars = input.ToCharArray();

        var palindromes = new List<string>();

        for(int i = 0; i < inputChars.Length; i++)

        {

            var letter = inputChars[i];

            var index = input.IndexOf(letter, i);           

            while(index > -1)

            {

                var substring = input.Substring(i, index - i + 1);

                char[] charArray = substring.ToCharArray();

                Array.Reverse(charArray);

                var reversed = new string(charArray);

                if (substring == reversed)

                {

                    palindromes.Add(substring);

                }

                index = input.IndexOf(letter, index + 1);           

            }

        }

        var longest = palindromes[0];

        foreach(var palindrome in palindromes)

        {

            if (palindrome.Length > longest.Length) longest = palindrome;

        }

        return longest;

    }

}

​

​

​

​