You've practiced foundational system design problems.
URL Shortener: Read-heavy workload. Cache aggressively. Short code generation is the interesting part.
Rate Limiter: Token bucket or sliding window log. Distributed rate limiting needs coordination.
Key-Value Store: Partitioning, replication, consistency trade-offs. CAP theorem in practice.
Pastebin: Similar to URL shortener with larger payloads. Object storage for content, metadata in DB.
Unique ID Generator: Snowflake-style IDs for distributed systems. Time-ordered, no coordination.
Leaderboard: Redis sorted sets for O(log N) ranking operations. Handle ties with composite scores.
These patterns recur in complex designs. Master them before tackling harder problems.