Then what does "with the same multiplicities" mean? Isn't "multiplicities" the amount? I'm not native English speaker, so excuse me if I misunderstood it.

...whether the two arrays have the "same" elements, with the same multiplicities.

the same multiplicities - doesn't it mean the same amount for each num and its square?

The description says:

If a or b are nil (or null or None), the problem doesn't make sense so return false.
If a or b are empty the result is evident by itself.

"Evident by itself" must mean "the problem doesn't make sense so return false" as well.
Otherwise shouldn't we return true for NULL arrays?

Please make the description clear.
I solved correctly, it's the the description is wrong.

Why return true for empty array?
The description says:

If a or b are nil (or null or None), the problem doesn't make sense so return false.
If a or b are empty the result is evident by itself.

"Evident by itself" must mean "the problem doesn't make sense so return false" as well.
Otherwise shouldn't we return true for NULL arrays.

Please make the description clear.