## [409\. Longest Palindrome](https://leetcode.com/problems/longest-palindrome/)

Given a string `s` which consists of lowercase or uppercase letters, return _the length of the **longest palindrome**_ that can be built with those letters.

Letters are **case sensitive**, for example, `"Aa"` is not considered a palindrome here.

**Example 1:**

**Input:** s = "abccccdd"
**Output:** 7
**Explanation:** One longest palindrome that can be built is "dccaccd", whose length is 7.

**Example 2:**

**Input:** s = "a"
**Output:** 1
**Explanation:** The longest palindrome that can be built is "a", whose length is 1.

**Constraints:**

- `1 <= s.length <= 2000`
- `s` consists of lowercase **and/or** uppercase English letters only.

回傳最長迴文的長度

注意題目的字母是 case sensitive，`aA` 不被視為迴文

class Solution {
public:
    int longestPalindrome(string s) {
        int mid = 0, res = 0;
        unordered_map<char, int> freq;
        for(auto& c : s) ++freq[c];
        for(auto& f : freq) {
            res += f.second;
            if(f.second % 2 == 1) {
                --res;
                mid = 1;
            }
        }
        return (mid == 1) ? ++res : res;
    }
};

:::success
- 時間複雜度：$O(N)$
- 空間複雜度：$O(N)$
:::