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

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
 
Please complete the following python code precisely:
```python
class Solution:
    def calculate(self, s: str) -> int:
```