Thanks, glad you learned something here

This solution is so nice. Learned a lot from it.

this one should be quicker than '\n'.join(). nice try!

Test in Python says to return None, but test written to look for ''.

Thanks it's easy to adapt this algorithm to scan only a third of all numbers instead of an half.

Correct, I should have bounded my compliment properly :)

except 2... ;)

mind = 'blown'

I don't see how this code rejects a list that doesn't match the
nxn dimensional requirement.