2131. Longest Palindrome by Concatenating Two Letter Words

LongestPalindrome.cs

public class Solution {
  public int LongestPalindrome(string[] words) {
    var freq = new Dictionary<string, int>();
    var palindromes = new HashSet<string>();
    var counter = 0;

    foreach(var word in words){
      if(!freq.ContainsKey(word))
        freq[word] = 0;
      freq[word]++;
      if(word[0] == word[1])
        _ = palindromes.Add(word);
    }

    foreach(var palindrome in palindromes){
      if(freq[palindrome] == 1)
        continue;
      if(freq[palindrome] % 2 == 0){
        counter += 2 * freq[palindrome];
        freq.Remove(palindrome);
      }
      else {
        counter += 2 * (freq[palindrome] - 1);
        freq[palindrome] = 1;
      }
    }
    foreach(var kv in freq){
      if(kv.Key[0] == kv.Key[1]){
        counter += 2;
        break;
      }
    }
    foreach(var palindrome in palindromes)
      if(freq.ContainsKey(palindrome))
        freq.Remove(palindrome);
    
    foreach(var word in words){
      var reversed = $"{word[1]}{word[0]}";
      if(freq.ContainsKey(word) && freq.ContainsKey(reversed)){
        var min = Math.Min(freq[word], freq[reversed]);
        counter += min * 4;
        freq.Remove(word);
        freq.Remove(reversed);
      }
    }
    
    return counter;
  }
}