with what protocole? What inputs? how were you measuring the time?

Without this kind of information, any perf tests is worthless (and as a matter of fact, yours sounds pretty suspicious: I bet you tried in the trainer? Then you're not measuring anything except the docker which is using most of those times => not reliable)

note: spoiler flag...

Oh my god, my solution with 'undefined' was accepted, in that case, we need to feedback more strict cases for this kata.

The description is talking about JS. In Python you have to fill it with None's.

if v is ommited, fill the array with undefined

as mentioned in the details' description above, when called with prefill(1), should return ['undefined'], but there seemds to be no testcases of this kind.

That's an old code of mine. An ideal solution would not even iterate over the whole array like the one I've just submitted.

your code is also O(n) and much cooler, exactly what i was seeking in the solutions. thumbs up

this solution has O(n) complexity