409. Longest Palindrome

set, map, array的基本用法，优先使用array; palindrome性质(允许存在一个字符为奇数的字符串)

1.set:

public int longestPalindrome(String s) {
    Set<Character> set = new HashSet<>();
    int count = 0;

    for (char a : s.toCharArray()) {
        if (set.contains(a)) {
            set.remove(a);
            count++;
        } else {
            set.add(a);
        }
    }
    return set.isEmpty() ? count * 2 : count * 2 + 1;
}

2.array:

public int longestPalindrome(String s) {
    int[] count = new int[128];
    for (char c : s.toCharArray()) {
        count[c]++;
    }

    int res = 0;
    int odd_count = 0;
    for (int value : count) {
        if (value % 2 == 1) odd_count++;
        res += value;
    }
    return odd_count > 0 ? res - odd_count + 1 : res;
}