Returning empty array for empty timetable makes no sense whatsoever. It should return empty string.

Test cases starting from the 5th are vulnerable to input modification.

"You may be given an array only of Monday and Friday with the same hours, but they shouldn't be concatenated." --Why? And what about such a situation should mean nothing would be concatenated? I haven't put any logic in that handles this, and it passed just fine.

"If the array is empty, it just returns an empty array." --Seems strange to return a different format type (other than something like null). I would've thought we'd return an empty string, as if it were a blank sign for the working hours.