I'm not educated in programing, but isn't that n^3 becouse of .sum? slice is just "one time" (or how I should call it) opearation.

You're right, missed that slice and just assumed it was a straight iteration over the original... even slower!

It's O(n^3) due to a slice inside the 2-level for loop.

This is still O(n^2) - the worst case is when there are no negative numbers.

You don't have to return early in the empty array case ;) but other than that, this is the most concise and readable linear-time solution.