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.

题目的大体意思：

给定一个字符串包含大小写，用字符串中的字符组成一个回文串求符合要求的回文串的最大长度。

      利用HashSet中去重复的特性，出现偶数重复的的话，就从set中remove出去，出现一次或者奇数次数的话，就会添加到HashSet中，根据String中总长度和set的长度判断回文的长度。

 public int longestPalindrome(String s) {
        if(s==null|| s.length()==0)
        return 0;
        if(s.length()==1)
        return 1;
        HashSet<Character> set=new HashSet<Character>();
         
          for(int i=0;i<s.length();i++){
              if(set.contains(s.charAt(i))) {
                  //如果重复就 
                    set.remove(s.charAt(i));
                   
                } else {
                    set.add(s.charAt(i));
                }
               
            }
          
           
            if(set.size()>0)
            return s.length()-set.size()+1;
            else
            return s.length()-set.size();
    }