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

Given a string of English letters s, return the greatest English letter which occurs as both a lowercase and uppercase letter in s. The returned letter should be in uppercase. If no such letter exists, return an empty string.
An English letter b is greater than another letter a if b appears after a in the English alphabet.
 
Please complete the following python code precisely:
```python
class Solution:
    def greatestLetter(self, s: str) -> str:
```