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

An n x n matrix is valid if every row and every column contains all the integers from 1 to n (inclusive).
Given an n x n integer matrix matrix, return true if the matrix is valid. Otherwise, return false.
 
Please complete the following python code precisely:
```python
class Solution:
    def checkValid(self, matrix: List[List[int]]) -> bool:
```