using System.Collections.Generic;
using System.Linq;
using static PalindromeChecker;

static class PalindromeChecker {
    public static string LongestPalindrome(IEnumerable<string> strings) {
        return strings.Select(LongestPalindromeSubstring).Max();
    }

    public static bool IsPalindrome(string s) {
        return s == new string(s.Reverse().ToArray());
    }

    public static string AsString(this IEnumerable<char> source) {
        return new string(source.ToArray());
    }

    public static string LongestPalindromeSubstring(string s) {
        var end = s.Length;
        var start = 0;
        var longestString = s.Take(1).AsString();
        while ((end-start) > longestString.Length) {
            while ((end - start) > longestString.Length) {//
                var subs = s.Skip(start).Take(end--).AsString();
                if (IsPalindrome(subs)) {
                    longestString = subs;
                }
            }
            start++;
            end = s.Length;
        }
        return longestString;
    }
}

public class Test {
	public static void Main() {
		var list = new List<string> { "aabbaa","carracecar", "aabbbabab" };
		System.Console.WriteLine(LongestPalindrome(list));
	}
}