Set and Dictionary complexity. Advantage over Array?

Junior PRO

The gist

Set is an unordered collection of unique elements; Dictionary holds key–value pairs with unique keys. Both are hash tables, hence:

Lookup (contains, subscript), insertion, removal — O(1) on average.
Worst case — O(n) (all keys in one bucket), which in practice doesn't happen thanks to SipHash with a random seed.
Iteration — O(n), order is undefined and changes between runs.
Growing the table (rehash) — O(n), but amortized; reserveCapacity avoids it.
Set operations (union, intersection, subtracting) — O(n + m).

A Set element and a Dictionary key must be Hashable; the dictionary value can be anything.

The rest of this answer, with code and interview follow-ups, is in the DevPrep app.