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

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.
If there are fewer than k characters left, reverse all of them. If there are less than 2k but greater than or equal to k characters, then reverse the first k characters and leave the other as original.
 
Please complete the following python code precisely:
```python
class Solution:
    def reverseStr(self, s: str, k: int) -> str:
```