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

A string s is nice if, for every letter of the alphabet that s contains, it appears both in uppercase and lowercase. For example, "abABB" is nice because 'A' and 'a' appear, and 'B' and 'b' appear. However, "abA" is not because 'b' appears, but 'B' does not.
Given a string s, return the longest substring of s that is nice. If there are multiple, return the substring of the earliest occurrence. If there are none, return an empty string.
 
Please complete the following python code precisely:
```python
class Solution:
    def longestNiceSubstring(self, s: str) -> str:
```