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Recognizing problems that need different approaches.
Two pointers fails when:
Unsorted without monotonic property: Need hash maps if sorting changes answer.
All pairs needed: Can't avoid .
Non-monotonic constraints: Product with negatives may miss solutions.
Pointer movement unclear: If moving doesn't narrow search space, rethink.
Alternatives: Hash maps for unsorted two-sum, sliding window for subarrays, binary search for jumps.
