LeetCode #409 - Longest Palindrome

题目描述：

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.

利用字符串中所有的字母，求问最长可以构造一个多长的回文字符串。利用哈希表统计字符串中所有字母次数，偶数个自然可以构成回文字符串，奇数个只能取其次数减一用来构建回文字符串，但是只要有一个奇数次的字母，就可以以它为中心，那么整体的长度还可以再加一。

class Solution {
public:
    int longestPalindrome(string s) {
        unordered_map<char,int> hash;
        for(int i=0;i<s.size();i++)
        {
            if(hash.count(s[i])==0) hash[s[i]]=1;
            else hash[s[i]]++;
        } 
        unordered_map<char,int>::iterator it;
        int even_sum=0;
        int odd_sum=0;
        bool flag=false;
        for(it=hash.begin();it!=hash.end();it++)
        {
            if((*it).second%2==0) even_sum+=(*it).second;
            else 
            {
                odd_sum+=(*it).second-1;
                flag=true;
            }
        }
        if(flag) return even_sum+odd_sum+1;
        else return even_sum;
    }
};