array reduce transformation 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
Look for understanding of sequential operations and the effect of passing intermediate results to the reducer function.
Solve rhythm
State the active state and invariant first, explain how each update preserves them, then pressure-test with counterexamples.
Most common miss
Forgetting to handle empty arrays and returning the correct initial value.
Recognition signals
- Look for understanding of sequential operations and the effect of passing intermediate results to the reducer function.
- Test if the candidate accounts for edge cases like empty arrays properly.
Solve flow
- 1. Define the active state/window.
- 2. Update state while preserving invariants.
- 3. Validate with edge-heavy examples.
Common misses
- Forgetting to handle empty arrays and returning the correct initial value.
Recommended Ladder
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