Because this implementation needs FIFO behavior. Though, kotlin lists have first() and last() methods that may provide same behavior original Java lists didn't. Moreover, some would say that push() and pop() is semantisemantically clearer.

yep)

But that's being super-pedantic since you're returning a list and not an array. It's a shared description. Imo returning a tuple in this context makes more sense for Python.

Even though they say return array not tuple test cases are passed.
Here you return tuple instead of array

Why simple list will not do the job? Don't understand what's the purpose of using Stack or ArrayDeque here...

It would be better to use ArrayDeque instead of java.util.Stack as suggested in multiple articles. By the way quite good and clear solution)

You're basically right, my bad

Ok, what if we have more than 3 'WUB' subsequently? :)