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

Given a string s and an array of strings words, determine whether s is a prefix string of words.
A string s is a prefix string of words if s can be made by concatenating the first k strings in words for some positive k no larger than words.length.
Return true if s is a prefix string of words, or false otherwise.
 
Please complete the following python code precisely:
```python
class Solution:
    def isPrefixString(self, s: str, words: List[str]) -> bool:
```