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

Given a string columnTitle that represents the column title as appears in an Excel sheet, return its corresponding column number.
For example:

A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28 
...

 
Please complete the following python code precisely:
```python
class Solution:
    def titleToNumber(self, columnTitle: str) -> int:
```