two pointers 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
The prompt hints at a reusable state-update rhythm.
Solve rhythm
State the active state and invariant first, explain how each update preserves them, then pressure-test with counterexamples.
Most common miss
The state is underspecified, so maintenance falls apart in implementation.
Recognition signals
- The prompt hints at a reusable state-update rhythm.
- Brute force is too slow, but the structure exposes a maintainable invariant.
- Follow-ups usually target edges, state transitions, and complexity.
Solve flow
- 1. Define the active state/window.
- 2. Update state while preserving invariants.
- 3. Validate with edge-heavy examples.
Common misses
- The state is underspecified, so maintenance falls apart in implementation.
- The template is memorized, but the update rule cannot be justified.
- Happy-path examples pass, then edge cases and complexity follow-ups break the answer.
Problem bank
two pointers pattern bank
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Progressive pattern bank
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