func longestPalindrome(s string) (l int) {
	freq := make([]int, 256)
	for _, r := range s {
		freq[r]++
	}
	hasOdd := false
	for i := 'a'; i <= 'z'; i++ {
		l += freq[i]
		if freq[i]&1 == 1 {
			l--
			hasOdd = true
		}
	}
	for i := 'A'; i <= 'Z'; i++ {
		l += freq[i]
		if freq[i]&1 == 1 {
			l--
			hasOdd = true
		}
	}
	if hasOdd {
		return l + 1
	}
	return
}