A harder test

Given how well a simple test did, I’m going to rerun a challenge I posted 2 years ago. Answers are welcome (that time only one answer came in, and it was wrong :()

Produce a zig-zag array.

A zig-zag array is a square array of the first N2 integers, where the numbers run from lowest to highest, laid out in a zig-zag along the anti-diagonals of the matrix (see the figure).

For example, if N is 5, the program must produce this array:

0 1 5 6 14
2 4 7 13 15
3 8 12 16 21
9 11 17 20 22
10 18 19 23 24

No cheating. Don’t look up the answer on Google or Stack Overflow. Think it through and write it in your favorite programming language; the more exotic the language the better, and there might even be a prize ;)

• This may look like a brain teaser, and a rather useless one, but the truth is that this kind of array is used in the JPEG image compression algorithm.