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Design a human activity recognition system (from sensors or video).

Recognizing walking, driving, or a fall from a sensor stream is windowed time-series classification, usually on a battery-bound device. The signal is windowing, temporal smoothing, and the on-device constraints most candidates skip. Here is the design.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

Recognizing walking, driving, or a fall from a sensor stream is windowed time-series classification, usually on a battery-bound device. The signal is windowing, temporal smoothing, and the on-device constraints most candidates skip. Here is the design.

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Next in this trackExplain matrix factorization for recommendation, and how it compares to modern approaches.Next in this trackDesign an event recommendation system (events, jobs, or other time-sensitive items).Next in this trackDesign a large-scale text classification system (e.g. news categorization or topic tagging).
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