The solution to the problem is not very efficient nor helpful. Why is there O(n) extra space? It's like the monotonic stack pattern.

Typically result space is not calculated in the Big O, so that's why res is not included as space complexity. You can note this to your interviewer if anyone isn't onboard.

    def leftRightDifference(self, nums: List[int]) -> List[int]:
        
        leftsum, rightsum, res = 0, sum(nums), []
        for n in nums:
            rightsum -= n
            res.append(abs(leftsum - rightsum))
            leftsum += n
        return res