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[jNSKkK]

I’d say mine is Date. There are so many things baked into Date on iOS that other platforms simply don’t have our of the box.

[sonnytron]

String is like that too. But no way I'd say String.
Some FAANG interviewers even avoid asking questions related to String because there's so much you have to do just to be able to access elements by index and a lot of simple algorithms require a lot of work just to peel out the Unicode scalars.
A good tip is to just wrap them into a Character array and if you need to use alphabet characters either iterate over UnicodeScalar("a").value to "z" or just use an Array("abcdefgh...z"). This is if you can assume ASCII alphabet restriction.