If this is given in Javascript, I'd rate the promise solution higher since a Javascript job involves a ton of promisified async code like this. If this is given in C or pseudo-code, then for sure a good ol stack+flag+loop is looking good enough.

> without considerable effort

Are you sure? I'm not a javascript programmer and it took less than 2 minutes to comprehend. If you think this takes considerable effort to comprehend then I don't think you actually tried.