Editorial for Another Contest 4 Problem 4 - Another Isosceles Right Triangle

Remember to use this editorial only when stuck, and not to copy-paste code from it. Please be respectful to the problem author and editorialist.
Submitting an official solution before solving the problem yourself is a bannable offence.

The function you are to implement is to return the $n^\text{th}$ ~n^\text{th}~ square, as clued by the problem statement and the titles to problems 3 and 4.