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.

思路：

用一个map记录每一个字符出现的次数，偶数次可以直接用来形成回文串，奇数次需要减去1，然后再最后结果上加上1个字符能够形成最长的回文串。

int longestPalindrome(string s) 
{
  if(s.size()<=1)return s.size();
  map<char,int>mp;
  for(char ch : s)
  {
    mp[ch]++;
  }
  int length = 0,odd = 0;
  for(auto it = mp.begin();it!=mp.end();it++)
  {
    if(it->second %2 ==1)
    {
      odd = 1;
      length+= it->second -1
    }
    else length+= it->second ;
  }
  length +=odd;
  return length;
}

[leetcode-409-Longest Palindrome]