Design it. Implement it. Prove it. 5 problems
Same problem, escalating constraints. Stage 1: naive solution. Stage 2: scaled. Stage 3: prove correctness with a conservation invariant — the senior filter. Each stage unlocks after you submit the previous.
Low-level design interview practice, answered
Low-level design (LLD) interviews ask you to design a working class-level solution, then hold it up as constraints escalate — concurrency, scale, failure modes — the way a senior loop keeps pushing after the first working answer. Senior Loop's Build It problems are staged: you start with a correct baseline and move through tightening constraints toward the senior-level version, proving correctness with a conservation invariant instead of an eye-test.
What is a low-level design interview?+−
You're asked to design classes, interfaces, and data structures for a system — a rate limiter, a parking lot, an in-memory cache — then defend and evolve that design as the interviewer adds constraints.
How is LLD different from system design (HLD)?+−
System design (HLD) is about services, databases, and scale at the architecture level. LLD is one level down — the actual classes, interfaces, and object interactions that make a single component correct and extensible.
What makes Build It different from a typical LLD prompt?+−
Each problem escalates through stages instead of stopping at a first working answer. Stage 3 is the senior filter, and you prove correctness with a conservation invariant rather than eyeballing the output.
Is low-level design practice free?+−
Yes — the first Build It problem is free with no card required.