Number of Recent Calls (opens on LeetCode in a new tab)
Count calls in the inclusive 3000 ms window and test a request exactly at its oldest allowed timestamp.
LeetCode design exercises
12exercises
Practice the rules each object must preserve.
Metadata checked 2026-10-25. LeetCode public problem list
12 matching exercises
State and small APIs
Start with one object and a small set of operations. Test capacity limits and time boundaries after each call.
Design Parking System (opens on LeetCode in a new tab)
Track spaces separately by car type and reject a car when its matching capacity reaches zero.
Design Circular Queue (opens on LeetCode in a new tab)
Preserve front, rear and item count as indices wrap around; test empty and full states.
Design Authentication Manager (opens on LeetCode in a new tab)
Expire tokens before actions at the same timestamp, and renew only tokens that are still live.
Application models
Represent history, journeys or stored values, then keep each update consistent with the model.
Design Browser History (opens on LeetCode in a new tab)
Move within history bounds and discard forward history after visiting a new page.
Design Underground System (opens on LeetCode in a new tab)
Match check-ins to check-outs and keep trip count and total duration for each directed station pair.
Time Based Key-Value Store (opens on LeetCode in a new tab)
Store timestamped values per key and return the newest value at or before each query time.
Design Twitter (opens on LeetCode in a new tab)
Maintain follow relationships and merge recent posts into a feed of at most ten items.
Design Memory Allocator (opens on LeetCode in a new tab)
Allocate the leftmost free block of the requested size, then free every block with the matching memory ID.
Caches and prefix indexes
Keep lookup and eviction rules consistent as keys, values and usage change.
LRU Cache (opens on LeetCode in a new tab)
Keep lookup and recency order consistent; evict the least recently used key when capacity is exceeded.
Your LRU cache now holds redirect mappings. What happens when a link expires, the process restarts or several readers request the same missing code?
Apply this in System DesignLFU Cache (opens on LeetCode in a new tab)
Maintain frequency and recency together; break equal-frequency eviction ties by least recent use.
Implement Trie (Prefix Tree) (opens on LeetCode in a new tab)
Distinguish complete stored words from prefixes, and test shared paths after inserting another word.