Yes, good point.

You are checking the lengths 4 times for no reason

How is this best practices

Why is this upvoted as best practices?

No, they are inside the class so you can use them all you want. Being private just means they can't be accessed by instantiated objects.

This form int mx[n][n] is a special case of int *mx and both are linear memory!
But the first form offsets automatically contrast to the second where you have to compute offsets yourself.
So with this example:

    int mx[5][5] = {
        1, 11, 111, 1111, 11111,
        2, 22, 222, 2222, 22222,
        3, 33, 333, 3333, 33333,
        4, 44, 444, 4444, 44444,
        5, 55, 555, 5555, 55555
    };
    int *x = mx;

mx[2][3] is equivalent to *(x + 2 * 5 + 3)

And answering your question... int * is returned because you cannot return int[n][n] from the function!

"And what about the number 1010000000000000000000000000000000000000000000000000000000000000000 ?"

How often do you use that large number?

In C, why would you pass 2d arrays and want to return a 1d pseudo-multidimensional array? You don't even specify it.

Just habit.

For function ptr func I think (*func)(...) is same as func(...) isn't it?

???

why are you dereferencing fun?

why would you even use recursion in this problem

why

And what about the number 1010000000000000000000000000000000000000000000000000000000000000000 ?

Best practice is what people voted best practice. You must get used to it, and if you don't agree, you are free to vote for whatever you think is good practice.