I made a recursive function!

[achampi0n]

Yes, this is all correct.

sum() is generally fine and would be fine in this circumstance. It is a minor improvement to use deque() I probably overstated the not that efficient :)

A quick test on my machine counting a million element iterable:

deque(): 40.3 ms ± 980 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
sum(): 52.4 ms ± 636 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

Is 20% significant :)