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

You are given a string array words and a string s, where words[i] and s comprise only of lowercase English letters.
Return the number of strings in words that are a prefix of s.
A prefix of a string is a substring that occurs at the beginning of the string. A substring is a contiguous sequence of characters within a string.
 
Please complete the following python code precisely:
```python
class Solution:
    def countPrefixes(self, words: List[str], s: str) -> int:
```