`HashMap<K, V>` needs keys implementing Hash + Eq. `entry()` gives in-place insert-or-update, and `get` returns Option<&V>. HashSet is a HashMap with unit values; BTreeMap keeps keys sorted when ordering matters.
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Collections · easy
17. HashMap and HashSet
Key-value and unique-set storage.
Check your understanding
1. Which API inserts-or-updates in one lookup?
2. Which map keeps keys sorted?