This is almost too easy. It works though, nice.

I solved it with using regexp

Not a suggestion.

this problem is painful

What about the case the person buys 3n-1 mangoes and get 1 mango free?

Iterating one time is orthogonal to using exceptions for control flow, so it doesn't really answer the question. I think using exceptions for control flow is considered less evil in Python than in other languages, although it's still questionable. But best practices votes here are so random that no one knows if it's best practices at all )

yes, this way you traverse the list only one single time

Duplicate of https://www.codewars.com/kata/54492291ec342c4a440008c5.

Can you please be more careful when approving a kata next time? This kata has several issues and design flaws.

Also this is an issue.

The only cases in which you would not be able to produce combinations is that in which arr is null or has a length less than k

What about k < 0? If you're testing k > arr.length, I don't see why this is not included.

The case arr === null is not tested.

(It's also a kinda pointless input validation, so I'd suggest dropping it completely.)