Design the operation logic for an elevator system. Start with basic scheduling for a single building: how do you handle up/down call requests from floors, how does the elevator decide its direction and stops? Then extend to multi-elevator allocation — how do you assign a request to one of N elevators efficiently? Follow-up: the building has many floors (high-rise scenario). How do you partition floors across elevators? Discuss the sky lobby concept (express elevators to a mid-level transfer floor, then local elevators for upper zones) and its tradeoffs.
Naive approach with serious trade-off — being authored.
Solid baseline with reasonable trade-offs — being authored.
Production-grade approach with explicit trade-off rationale — being authored.
Naive approach with serious trade-off — being authored.
Solid baseline with reasonable trade-offs — being authored.
Production-grade approach with explicit trade-off rationale — being authored.
Cover happy path. Clarify scope. Identify the obvious bottleneck. Pick a reasonable storage and reasonable scaling approach.
All of the above plus: explicit failure handling, durability vs latency trade-offs, choose the right batching/caching strategy, articulate why.
All of the above plus: organizational concerns (rollout, migration, on-call), quantitative analysis, multi-region considerations, what could go wrong with the proposed solution at 10x scale.
Common mistakes: High-rise partitioning answer becoming disorganized/scattered without a clear framework; Not defining a concrete cost function for multi-elevator assignment; Forgetting direction compatibility when assigning requests to moving elevators
What passers do: Progressively structured answer: single elevator → multi elevator → high-rise; Articulating a concrete scheduling algorithm (LOOK/scan) with reasoning; Proactively mentioning sky lobby and zone tradeoffs before being prompted
Why people fail: High-rise section answered in a scattered, unstructured way with no clear framework; Staying only at a conceptual level without concrete data structures or algorithm choices
Edge cases probed: High-rise with very many floors requiring zone partitioning; Sky lobby design and transfer floor placement; Multiple elevators — how to avoid all elevators clustering at same floor
Alternative approaches: FCFS (First-Come-First-Served) (Simple to implement but leads to high average wait times and erratic elevator movement; not realistic for production.); Destination Dispatch (Group Control) (Passengers enter destination floor at lobby; system groups passengers going to same/nearby floors into one car. Reduces stops and travel time but requires special input hardware and more complex allocation logic.); Predictive / ML-based scheduling (Uses historical demand patterns (rush hours, lunch peaks) to pre-position elevators. Higher throughput at scale but complex to build and validate.)