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：

用一個map記錄每一個字符出現的次數，偶數次可以直接用來形成回文串，奇數次需要減去1，然後再最後結果上加上1個字符能夠形成最長的回文串。

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]