Thanks for the explanation. Mathematics is the most powerful tool to simplify problems.

Thanks a lot for the explanation <3.

I see a lot of challenges are mathematical stuff. If one doesn't realize the math concept behind it, one may take a long time guessing stuff, getting nowhere, or doing a poor job with the solution.

I believe, for instance, that this very problem is related to Arithmetic Sequences and Sums.

I can see why that would be a concern, however if you look at both left and right, they are shortened with slice. Since left + right = arr, then left.reduce + right.reduce = arr.reduce, at least as far as the total length and amount of items reduced.