using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var tests = new List<string>()

        {

            "",

            "1",

            "123",

            "1221",

            "12321",

            "3456543",

            "34565431",

        };

        foreach (var test in tests)

        {

            Console.WriteLine(test + (IsPalindromeString(test) ? " is " : " is NOT ") + " palindrome string");

            var longestPalindrome = FindLongestPalindromeString(test);

            if (string.IsNullOrEmpty(longestPalindrome))

            {

                Console.WriteLine(test + " has NO sub-palindrome string");

            }

            else

            {

                Console.WriteLine("the longest palindrome string of " + test + " is " + longestPalindrome);

            }

        }

    }

    public static bool IsPalindromeString(string s)

    {

        if (string.IsNullOrEmpty(s) || s.Length <= 1) return false;

        var head = 0;

        var end = s.Length - 1;

        while (head < end)

        {

            var headChar = s[head];

            var endChar = s[end];

            if (headChar != endChar)

            {

                return false;

            }

            head++;

            end--;

        }

        return true;

    }

    public static string FindLongestPalindromeString(string s)

    {

        if (string.IsNullOrEmpty(s)) return null;

        var subStringLength = s.Length;

        while (subStringLength > 0)

        {

            for (var i = 0; i <= s.Length - subStringLength; i++)

            {

                var subString = s.Substring(i, subStringLength);

                if (IsPalindromeString(subString))

                {

                    return subString;

                }

            }

            subStringLength--;

        }

        return null;

    }

}