409. Longest Palindrome

class Solution {
    public int longestPalindrome(String s) {
        int longestPalindrome = 0;
        Map<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            map.put(c, map.getOrDefault(c, 0) + 1);
        }
        boolean flag = false;
        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
            if (entry.getValue() % 2 == 0) {
                longestPalindrome += entry.getValue();
            } else {
                if (!flag) {
                    longestPalindrome += entry.getValue();
                    flag = true;
                } else {
                    longestPalindrome += entry.getValue() - 1;
                }
            }
        }
        return longestPalindrome;
    }
}