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

Given a string s, partition the string into one or more substrings such that the characters in each substring are unique. That is, no letter appears in a single substring more than once.
Return the minimum number of substrings in such a partition.
Note that each character should belong to exactly one substring in a partition.
 
Please complete the following python code precisely:
```python
class Solution:
    def partitionString(self, s: str) -> int:
```