You almost never build a hash map yourself — every language ships a fast one. This lesson is the toolkit: the same operations in all four languages, the one idiom you'll type more than any other (counting), and the traps that catch people in each language.
![A cheat sheet. JavaScript Map: new Map(), set, get which gives undefined if missing, get with ?? 0 for a default, has, delete, size, and for-of over the map. Python dict: braces, m[k] = v, m[k] which raises KeyError if missing, m.get(k, 0), k in m, del, len, and items(). Java HashMap: put, get which gives null if missing, getOrDefault, containsKey, remove, size, and entrySet(). C++ unordered_map: m[k] = v, m.at(k) which throws if missing, count to check, erase, size, and a range-for over the map.](/drawings/e88dd284-16b1-4898-858e-db908bfb7956.svg)
All of these are O(1) on average, except looping, which visits every entry. For a set, use Set (JavaScript), set() (Python), HashSet (Java) or unordered_set (C++) — the same operations without values.
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