Given a string which consists of lowercase or uppercase letters, find the length of the longest palindromes that can be built with those letters.

This is case sensitive, for example"Aa"is not considered a palindrome here.

Note:
Assume the length of given string will not exceed 1,010.
Example:
Input:   "abccccdd"
Output:  7
Explanation:
One longest palindrome that can be built is "dccaccd", whose length is 7.

刚开始用hashmap 发现总是过不了那个变态testcase。 于是看了大神代码，用set 统计