OP solved it, closing

Empty arrays are not in the challenge. That said, it was an interesting challenge to address.

The reason some are removed is the flattening logic removes and second level and beyond empty arrays.

To remove the first level empty arrays, add a filter to the return value:

.filter { !(($0 as? Array<Any>)?.isEmpty ?? false) }

that should be clear from the type of the function, and in this way it is better readable

lmaoooo

I made a change in the way I pick my steps and it finally worked, haha!

I was about to give up and go to sleep, but it's your comment that gave me hope for a last try. ^_^

Thank you! :D

Yes, it's elegant. But at the same time, it needs to iterate all the range from zero to n. I think this way wastes a lot of time when n is a big number.

I second this.

There should be a clarification in the description that multiples of both 3 and 5 should be included just once in the sum.
For example, 30 = 5 * 6, but also 30 = 3 * 10. The same is for 15, 45, 60, etc. In the current tests those numbers go once in the sum.

Nice kata!

Nice kata, thank you!

Decided to use a different tack to finish with this kata. I learned a lot. It would be interesting to see a solution using Regular Expressions especilly since the kata is tagged with this category. Additionally, having spent significant time reading around this topic quite a lot, I couldn't quite see how to solve the kata incorporating RegEx into the solution.

Thank you for a great kata.

Thank you for your very thoughful reply. The snippet you sent goes a long way to explaining how the guard statement logic works here. If the guard statement is commented out, the recursion has no way to stop it. What is also a very useful takeaway is the acknowledgement that a return statement will halt the function except when it contains a recursive component. Then another mechanism may be required to stop execution e.g. the guard statement in this case.

This is a lovely solution. When I test it, empty arrays are returned when they are inserted in the input array. Interestingly, not all of the empty arrays are returned...

flattenTwoLevels([1, [], [2, 3],[4, 5, [6, 7, 8], 9, [], 10, [11, [12, [13, 14], 15], 16], 17], [], 18]) ->

[1, [], [2, 3], [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17], [], 18]