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

Given two sparse matrices mat1 of size m x k and mat2 of size k x n, return the result of mat1 x mat2. You may assume that multiplication is always possible.
 
Please complete the following python code precisely:
```python
class Solution:
    def multiply(self, mat1: List[List[int]], mat2: List[List[int]]) -> List[List[int]]:
```