Longest palindrome in a given string using LINQ

Question

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);
 Console.ReadKey();
 }

• tia: 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?
• callee.args: I tried with several palindromes found online. e.g ROTAVATOR ->9,dad->3,malayalam->9
• tia: and the case where it fails?
• callee.args: I'm not able to create one. :(
• tia: Well, so what's the problem?

Answer 1

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.

• callee.args: 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.
• tia: My bad. I fixed the bug already.