It runs the same as n%2 because compilers turn it into a bit test, it only runs faster with optimizer turned off. I used it because once you're familiar with bit ops, it becomes a self-explanatory idiom.

clever use of bitshifting, probably runs very fast!

Yes, if returning a static label. It points to program data segment.

Is it considered best practice just to return a char* like this without allocating memory with calloc or malloc?
Where is the pointer stored - on the stack or heap?

Great job! Imo modulo is more readable but bit AND is cool as hell.

Added

Thanks!

How come it is not necessary to allocate memory for the returned strings? I see that it works but is it a good practice?

How does it work?

one or more of the arrays provided were empty, although the description mentioned that this couldn't happen.

that's not what the description says.

Anyway, the ref solution was incorrect for empty arrays. Corrected.