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

Given an m x n binary matrix mat, return the number of special positions in mat.
A position (i, j) is called special if mat[i][j] == 1 and all other elements in row i and column j are 0 (rows and columns are 0-indexed).
 
Please complete the following python code precisely:
```python
class Solution:
    def numSpecial(self, mat: List[List[int]]) -> int:
```