Please solve the programming task below using a self-contained code snippet in a markdown code block.

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.
 
Please complete the following python code precisely:
```python
class Solution:
    def longestPalindrome(self, s: str) -> int:
```