Hash Map Pattern

The Hash Map pattern is one of the reusable techniques that experienced candidates spot instantly. Rather than memorising individual problems, strong interviewees recognise that a new question "is just Hash Map" and reach for the matching template. That recognition is exactly what this collection is designed to build.

We index 50 problems that use the Hash Map pattern (21 easy, 25 medium, 4 hard), each worked brute-force → optimal in Java and Python with line-by-line explanations. Solving them back-to-back is the fastest way to internalise when and how to apply Hash Map.

Pattern-first practice compounds: once Hash Map is automatic, you'll see it inside problems that never mention it by name.