What is the problem with modifying the inital array? For all intents and purposes your original array should be destroyed when using this I believe.

I must admit that I only care about that when it's part of the description/tests.

Here's the same solution, but this version doesn't modify the original array:
https://www.codewars.com/kata/reviews/5514e5bcdbc294a0d50003fe/groups/5c5dc840d40a5d00016acdf8

This solution has a side effect of modifying the initial array when num isn't something like { 9, 9, 9, 9, ..., 9 }

Won't work for a = {2, 5} and b = {9 , 16}