I can't see how it uses more sum() calls that the regular loop solutions provided.

Thanks for the feedback... I didnt real how sloppy was doing it! I was stuck on figuring out the distinction between left and right, so i was printing out the sum for every position until it clicked... I just didnt take the time refine it.

I just did it in Python and it seemed fine to me. I didn't notice any bugs or wrong return values.