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

Given a string s, return the maximum number of occurrences of any substring under the following rules:

The number of unique characters in the substring must be less than or equal to maxLetters.
The substring size must be between minSize and maxSize inclusive.

 
Please complete the following python code precisely:
```python
class Solution:
    def maxFreq(self, s: str, maxLetters: int, minSize: int, maxSize: int) -> int:
```