Find All Duplicates in an Array

The scenario

you are processing a stream of events and need fast memory of what has already happened — a seen value, a running count, or a reusable prefix state.

What you'll learn

• How to recognize the In-Place Index Mapping pattern from the problem statement.
• The BASIC Framework breakdown for this problem.
• A clean, interview-ready Python solution with time/space analysis.

Full video, code, and the workspace are part of PatternMastery Pro — 500 problems for one $9.99 payment.