group by core interview pattern Pattern
Pattern hubs are for building transferable solving frames. Learn the recognition signals first, then drill state definition, update rules, and edge explanation until the pattern feels stable.
Pattern brief
Recognize first
Ask candidate to explain how keys are generated and handled if duplicates occur.
Solve rhythm
State the active state and invariant first, explain how each update preserves them, then pressure-test with counterexamples.
Most common miss
Overwriting arrays for duplicate keys instead of appending elements.
Recognition signals
- Ask candidate to explain how keys are generated and handled if duplicates occur.
- Check if candidate preserves the original element references in the grouped arrays.
Solve flow
- 1. Define the active state/window.
- 2. Update state while preserving invariants.
- 3. Validate with edge-heavy examples.
Common misses
- Overwriting arrays for duplicate keys instead of appending elements.
Recommended Ladder
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