Traverse Matrix in Diagonal strips

Question

I thought this problem had a trivial solution, couple of for loops and some fancy counters, but apparently it is rather more complicated.

So my question is, how woul

Answer 1

I would shift the rows like so:

1  2  3  x  x
x  4  5  6  x
x  x  7  8  9

And just iterate the columns. This can actually be done without physical shifting.