using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    static void Main(string[] args)

    {

        var input = "aaaaaabbbabbbbabcbcbc";

​

        // Expected: Longest palindrome Sub String is: aaaaaa

        Console.Write($"Longest palindrome Sub String is: {FindLongestPalindromeSubstring(input)}");

    }

​

​

    private static string FindLongestPalindromeSubstring(string input)

    {

        var longest = 0;

        var limit = input.Length;

        for (var i = 0; i < limit; i++)

        {

            for (var j = limit-1; j > i; j--)

            {

                var forwardString = input.Substring(i, j - i);

                var reversedString = new string(forwardString.Reverse().ToArray());

                if (forwardString == reversedString && forwardString.Length > longest)

                {

                    return forwardString;

                }

            }

        }

        return "";

    }

}