complement lookup 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
Do you recognize that Two Sum only needs a complement existence check, not comparison against every later element?
Solve rhythm
State the active state and invariant first, explain how each update preserves them, then pressure-test with counterexamples.
Most common miss
Inserting the current value before checking its complement can incorrectly pair an element with itself when the target is twice that value.
Recognition signals
- Do you recognize that Two Sum only needs a complement existence check, not comparison against every later element?
- Can you explain why checking the hash map before inserting the current value avoids reusing the same index?
Solve flow
- 1. Define the active state/window.
- 2. Update state while preserving invariants.
- 3. Validate with edge-heavy examples.
Common misses
- Inserting the current value before checking its complement can incorrectly pair an element with itself when the target is twice that value.
Recommended Ladder
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