Yeah, that’s why I said :)

Well, that’s quite simple. You transform your integer into a list of integers. Then, each digit d satisfies 0 ≤ d ≤ 9. If you double it, you get 0 ≤ 2d ≤ 18. So if d > 4 or 2d > 9, you just have to sub 10 to get the right digit, and add 1. Which is -9. 16 - 9 == 1+6 == 7, for instance.

I’m surprised how people understand the sentence “add the digits of the doubled”. The solution only requires a sub, come on guys, no need for digits! :D

Yeah, I forgot what -10 + 1 is. :D