Module 42
Pattern Recognition Drills
Mixed problems with the labels hidden: read the input shape, the question and the constraints, name the pattern, then check yourself.
Inside a module you already know which pattern is coming. In an interview you don't. This module trains the missing skill: looking at an unfamiliar problem and deciding, in a minute or two, which family of techniques fits and why.
The lessons give you a checklist (input shape, question type, constraint size) and a decision map. The drill problems come from across the course with their patterns hidden: on each problem page, name the pattern before you press reveal. Every problem is still fully worked, so you can check your reasoning afterwards.
Best after: Advanced DP, Greedy Algorithms
Unlocks next: 43. The Core 100
Part 1
Learn the ideas
- 42.1From Clue to PatternAsk three questions in order: what shape is the input, what kind of answer is wanted, and how large is n. Together they narrow dozens of patterns to two or three.14 min
- 42.2Constraints Tell You the ComplexityAbout 10⁸ simple operations run in a second. Divide that by n and you know which complexities fit, and therefore which patterns are even possible.10 min
- 42.3How to DrillGive yourself two minutes to name the pattern and say why, ten more to sketch the approach, then check. Keep a log of misses and revisit them after 1, 3 and 7 days.8 min
Part 2
Drill: name the pattern
Mixed patterns, no labels. Read each problem, decide which pattern fits and why, then reveal.