These are classic low-level design problems, grouped by difficulty. Work through them with the LLD interview framework: scope, entities, class skeleton, implementation, extensions. Problems with a worked design in this section are linked.
Level 1: single class or simple state#
Good for practicing the framework itself. One orchestrator class and a clear state machine or rule check.
| Problem | Core skill |
|---|---|
| Vending Machine | State machine, inventory, change |
| Connect Four | Board modeling, win detection |
| Logging Service | Strategy pattern, level filtering |
| ATM System | State machine, validation |
| Shopping Cart | Pricing rules, discounts |
Level 2: several collaborating entities#
More entities and relationships, plus an allocation or traversal algorithm.
| Problem | Core skill |
|---|---|
| Parking Lot | Allocation by size, tickets, pricing strategy |
| Elevator System | Scheduling (SCAN), direction state |
| Package Locker System | Best-fit allocation, one-time codes |
| File System | Tree structures, path traversal |
| Inventory Management | Stock invariants, low-stock alerts |
| Notification System | Channels as strategies, fan-out |
| Library Management | Borrow/return lifecycle, fines |
| Hotel Management | Room booking, availability by date |
Level 3: concurrency and trickier invariants#
Expect the interviewer to push on race conditions, time and consistency.
| Problem | Core skill |
|---|---|
| Rate Limiter | Sliding window, per-user locking |
| Movie Ticket Booking | Seat locking, two-phase booking |
| Key-Value Store | TTL, eviction, transactions |
| Expense Splitter (Splitwise-style) | Balances graph, debt simplification |
| Ride-Sharing Service | Matching, trip state machine |
| Food Delivery System | Order lifecycle, assignment |
Key takeaways#
- Start with Level 1 problems to make the framework automatic.
- Level 2 problems reward clean relationships and a good allocation data structure.
- Level 3 problems are mostly about concurrency: find the check-then-act and protect it.