Almost the same solution as me. But I wonder why this doesn't get an error "list index out of range", when i+5 are > len(digits). Can someone explain please?

highly inefficient

cheater ;-)

Well since we are talking about words/sentences, they can be only so long, so I don't see why performance would play a huge part in this code. At least for the test cases both solutions are almost equal (sorted even being a bit faster).

True but you have to use floor division in that case, otherwise you will get an error for range doesn't work with floats

If you split for example "a__b" (double space instead of underscores) into a list, it will result in ["a", "", "b"].
When you join it later, there will be one space between "a" and the empty string "" - and another space between the empty string and "b". Therefore you end up keeping the double space like in the original string.

yes, it's called list comprehension. Without the brackets you will get an error.

It doesn't, but: "You may assume the parameter is non-negative."

what's wrong about this solution and which would be best practice in this case?