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

In English, we have a concept called root, which can be followed by some other word to form another longer word - let's call this word derivative. For example, when the root "help" is followed by the word "ful", we can form a derivative "helpful".
Given a dictionary consisting of many roots and a sentence consisting of words separated by spaces, replace all the derivatives in the sentence with the root forming it. If a derivative can be replaced by more than one root, replace it with the root that has the shortest length.
Return the sentence after the replacement.
 
Please complete the following python code precisely:
```python
class Solution:
    def replaceWords(self, dictionary: List[str], sentence: str) -> str:
```