Hash Map

What it is

A hash map is a data structure that stores key-value pairs.

The advantage is that you can look up a value by key without scanning everything.

In JavaScript

Map is the direct implementation:

const seen = new Map<number, number>()

seen.set(2, 0)
seen.has(2)
seen.get(2)

Plain objects can work for simpler cases, but Map is usually clearer when keys vary in type or you care about insertion order.

When to use it

Use Map when you need to:

• remember whether something already appeared
• keep a value associated with another value while iterating
• do fast lookups by key at any point in the algorithm

Why it shows up so often

A lot of optimization problems are really the same pattern:

you are iterating once and need quick memory of what you saw before.

That is how many O(n²) solutions become O(n).

Map is not the point. Fast memory during the pass is the point.