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

Given a string s, return the number of distinct substrings of s.
A substring of a string is obtained by deleting any number of characters (possibly zero) from the front of the string and any number (possibly zero) from the back of the string.
 
Please complete the following python code precisely:
```python
class Solution:
    def countDistinct(self, s: str) -> int:
```