409. Longest Palindrome

class Solution:
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: int
        """
        a_dict = {}
        for i in range (len(s)):
            if s[i] in a_dict:
                a_dict[s[i]] += 1
            else:
                a_dict[s[i]] = 1
        count = 0
        unit = 0
        for a_value in a_dict.values():
            if a_value % 2 == 0:
                count += a_value
            else:
                count += (a_value//2)*2
                unit = 1
        return count + unit