Given n and s, return a n-by-n matrix a of integers such that
sum(sum(a == rot90(a)))
is equal to exactly s. The integers should be chosen from the set 1 to n.
So for instance, if n is 2, and s is 2, you could return
a = [ 1 1 2 2 ]
since rot90(a) matches a in exactly one place.
For any n and s, a is of course not unique. I'm only checking to make sure the conditions are true.