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

You are given a string s. We want to partition the string into as many parts as possible so that each letter appears in at most one part.
Note that the partition is done so that after concatenating all the parts in order, the resultant string should be s.
Return a list of integers representing the size of these parts.
 
Please complete the following python code precisely:
```python
class Solution:
    def partitionLabels(self, s: str) -> List[int]:
```