16. Hashing

A HashMap stores key → value pairs, and a HashSet stores keys, both with O(1) average lookup. Inside, the key's hashCode picks a bucket; collisions go into a chain (Java turns long chains into trees).

The common patterns are: "have I seen this before?" (a set), "how many of each?" (a count map), "where did I see it?" (a value → index map), and "group by a canonical key". Remember that arrays used as keys compare by reference, so convert them to a String or List first.