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

The variance of a string is defined as the largest difference between the number of occurrences of any 2 characters present in the string. Note the two characters may or may not be the same.
Given a string s consisting of lowercase English letters only, return the largest variance possible among all substrings of s.
A substring is a contiguous sequence of characters within a string.
 
Please complete the following python code precisely:
```python
class Solution:
    def largestVariance(self, s: str) -> int:
```