Leetcode: Longest Palindrome

题目： 409. Longest Palindrome

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.

题意是找出字符串中可以组成回文字符串的最长长度。用HashSet来解决的话，HashSet是一个无序的集合。每出现一个重复的字符，计数器+1，并且将HashSet中已有的该字符移除。
这样，如果HashSet为空，说明，该字符串每个字符正好有偶数对，整个字符串便可以组成回文字符串，最长回文字符串的长度便是输入字符串的长度，即计数器count的2倍。如果HashSet不为空，说明，该字符串中某些个字符有奇数个，最终的最长回文字符串长度便是计数器count的2倍+1。

public class Solution {    public int longestPalindrome(String s) {        if(s == null || s.length() == 0)            return 0;        int count = 0;        HashSet<Character> hs = new HashSet<Character>();        for(int i = 0; i < s.length(); i++) {            if(hs.contains(s.charAt(i))) {                hs.remove(s.charAt(i));                count++;            }else {                hs.add(s.charAt(i));            }        }        if(hs.isEmpty()){            return 2*count;        }        return 2*count+1;    }}