4-liner, cheating on whitespace doesn't count.

Use spoiler flag.

The negative sign is not a digit, it is just a sign, therefore it shouldn't make a number to win

well mutation was not explicitly prohibited

You should considere it the other way around: if mutation isn't explicitly allowed, it's forbidden ;)

Errrr... Looking at it again, it's actually a pretty bad method! X-D
Mutation of the input. Pretty bad. Hopefully, I just did a second fork. This one would be worth the "best practices" the original solution earned (even if longer). ;)

Thx

(actually, the fork is faaaar better. ;) )

And another one here :-)

yes

EDIT: "one liner" in O(N) here (for fun)