It seems I broke the rule from the problem statement, but I can't remember to have read that by the time I implemented my solution (it was 8 yeas ago, by the way).

This is about what I was trying to do, but far more efficient.

The list is sorted. I think you can avoid using the set.

very clever way to put !

clever use of set

This does not fulfill the requirements at all.

Definitely not best practice. This fails to check if the correct amount of numbers are in each place, and I see what they were trying to do with the optional area code, but if you entered a phone number without an area code, it would fail, too, as you need a space before the phone number in that case.

ok

The problem statement said not to mutate the nodes. This does that, if I'm thinking correctly...

ohh!! ok cool. thank you

how do you slice into the list with non interger indices.
dont you get the following error:
TypeError: list indices must be integers or slices, not float

Yiah babe

I think so too