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Elevator System • Payment Gateway • Heap / Priority Queue • Tech: Cassandra
Algorithm
DSA
Heap / Priority Queue — Merge k Sorted Lists (LC 23)
Merge k sorted linked lists using a min-heap.
Example
Input: lists = [[1,4,5],[1,3,4],[2,6]] → Output: [1,1,2,3,4,4,5,6]
Where it shows up
Log mergers, multi-way merge in external sorting, streaming.
References
Low-level design
LLD
Elevator System
Requirements
- Multiple elevators; up/down requests; scheduling.
Expectations
- UML for Elevator, Request, Controller; strategy for scheduling.
- Simulate requests & movement.
Where it shows up
High-rise building operations, dispatch systems.
References
High-level design
HLD
Payment Gateway
Requirements
- Payment intents, confirm/capture, refunds; idempotency; webhooks.
Expectations
- PSP adapters, circuit breakers, retries with backoff.
- Audit logs; PCI isolation considerations.
Where it shows up
Fintech, marketplaces, subscriptions.
References
Today's deep-dive
Tech
Cassandra
Model a table for user_events with proper partition key; write queries; test CL=QUORUM reads/writes.
Where it shows up
Distributed databases, event sourcing, and high-availability systems where partition tolerance and tunable consistency are critical.
References