409\. Longest Palindrome

Easy

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 

**Example 3:**

**Input:** s = "bb"

**Output:** 2 

**Constraints:**

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