很简单的题
统计一遍奇数个数字母和偶数个数字母
加起来减掉奇数个数字母的个数
然后如果存在奇数个数的字母就加一

    public class Solution
    {
        public int LongestPalindrome(string s)
        {
            Dictionary<char, int> countChar = new Dictionary<char, int>();
            foreach (var ch in s)
            {
                if (countChar.ContainsKey(ch))
                    countChar[ch]++;
                else
                    countChar.Add(ch, 1);
            }
            int numOdd = 0;
            int countOdd = 0;
            int countEven = 0;
            int hasOdd = 0;
            foreach (var temp in countChar)
            {
                if (temp.Value % 2 == 0)
                {
                    countEven += temp.Value;
                }
                else
                {
                    numOdd++;
                    countOdd += temp.Value;
                    hasOdd = 1;
                }
            }
            return countEven + countOdd - numOdd + hasOdd;
        }
    }