In a one-hour coding test you have to find duplicates in a list of customer IDs and also count how often each ID appears. When would you use Swift's Set and when Dictionary?
For duplicate detection, where you only need to know whether you've seen it, insert into a Set and catch the moment inserted comes back false; when each key needs a value, like a count, use a Dictionary keyed by id with counts[id, default: 0] += 1. Both insert and look up in average O(1), so one pass is O(n).
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