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

You are given a string s consisting only of lowercase English letters. We call a substring special if it contains no character which has occurred at least twice (in other words, it does not contain a repeating character). Your task is to count the number of special substrings. For example, in the string "pop", the substring "po" is a special substring, however, "pop" is not special (since 'p' has occurred twice).
Return the number of special substrings.
A substring is a contiguous sequence of characters within a string. For example, "abc" is a substring of "abcd", but "acd" is not.
 
Please complete the following python code precisely:
```python
class Solution:
    def numberOfSpecialSubstrings(self, s: str) -> int:
```