Prerequisites

You should be comfortable with basic arrays, as hash maps often store array elements or their indices as values. Understanding key-value lookups and their $O(1)$ average time complexity is needed for recognizing when hash maps apply.

Hash map problems often reduce $O(n^2)$ brute force to $O(n)$ by storing previously seen elements for instant lookup. Practice the classic Two Sum problem to build this intuition before tackling harder patterns.