# Longest palindrome in a given string using LINQ

Problem:I need to write an algorithm that will return length of longest possible palindrome from a given string.

So if the input is aabbbccdfg Program output should be 7. //-->[cabbbac]

Can someone point it out if there are any problems with below code?

public static void GetLongestPalindromeLength()
{
var input = Console.ReadLine().ToCharArray();
var result = 0;
var countsByChar = input.GroupBy(c => c);
var oddCounts = countsByChar.Where(g => g.Count() % 2 != 0).Select(g => g.Count()).ToList();
var evenCounts = countsByChar.Where(g => g.Count() % 2 == 0).Select(g => g.Count()).ToList();
if (oddCounts.Any())
{
var max = oddCounts.Max();
result += max;
oddCounts.RemoveAt(oddCounts.FindIndex(e => e == max));
result += evenCounts.Sum();
result += oddCounts.Sum(e => e - 1);
}
else if (evenCounts.Any())
{
result += evenCounts.Sum();
}
Console.WriteLine(result);
}

• I am not sure what you are trying to do, but to me your algorithm is plain wrong if I understand the problem correctly. Could you provide some test cases? – tia Mar 28 '15 at 15:41
• I tried with several palindromes found online. e.g ROTAVATOR ->9,dad->3,malayalam->9 – callee.args Mar 28 '15 at 18:28
• and the case where it fails? – tia Mar 28 '15 at 18:35
• I'm not able to create one. :( – callee.args Mar 28 '15 at 18:37
• Well, so what's the problem? – tia Mar 28 '15 at 18:40

Well, I think your algorithm is correct, but the code is quite bloat. Your code suggest that the longest palindrome size can be calculated by sum of n / 2 * 2 where n is the count of distinct alphabet that exists more than once in the string, and plus 1 if there are any "oddCounts". So you can reduce the code to

    private static int GetLongestPalindrome(string value) {
var map = value.GroupBy(c => c);
int result = map.Sum(r => r.Count() / 2 * 2);
if (map.Any(r => r.Count() % 2 != 0))
{
result++;
}
return result;
}


n / 2 * 2 is the way to calculate for the nearest even number toward zero, and it equals to n - 1 where n is positive odd number.

• I think above code is not correct. e.g. for kkkkk output is 3 instead of 5. for MALAYALAM output is 7 instead of 9. – callee.args Mar 29 '15 at 3:39
• My bad. I fixed the bug already. – tia Mar 29 '15 at 3:49