Challenge Library
Coding Challenges
5,893 curated questions across every arena — browse by topic, then jump into a live 3-minute battle to put them to the test.
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1,575 challenges matching your filters
- Code AnalysisBeginner
Dict Merge 3.9+
code-analysisPythonlanguage - Code AnalysisBeginner
F-string Format Spec
code-analysisPythonlanguage - Code AnalysisIntermediate
Generator Exhaustion
code-analysisPythonlanguage - Code AnalysisHard
is vs == (int cache)
code-analysisPythonlanguage - Code AnalysisIntermediate
Lambda Late Binding
code-analysisPythonlanguage - Code AnalysisIntermediate
List Comp Scope
code-analysisPythonlanguage - Code AnalysisHard
Multiple Inheritance MRO
code-analysisPythonlanguage - Code AnalysisHard
Mutable Default Arg
code-analysisPythonlanguage - Code AnalysisIntermediate
nonlocal Keyword
code-analysisPythonlanguage - Code AnalysisBeginner
Reversed String
code-analysisPythonlanguage - Code AnalysisBeginner
Set Comprehension
code-analysisPythonlanguage - Code AnalysisBeginner
Star Unpacking
code-analysisPythonlanguage - Code AnalysisHard
String Interning
code-analysisPythonlanguage - Code AnalysisBeginner
Truthiness Edge Cases
code-analysisPythonlanguage - Code AnalysisIntermediate
Try Else Finally Order
code-analysisPythonlanguage - Code AnalysisIntermediate
Tuple Mutable Item
code-analysisPythonlanguage - Code AnalysisHard
Walrus in Comprehension
code-analysisPythonlanguage - Code AnalysisBeginner
Unequal Zip
code-analysisPythonlanguage - Code AnalysisHard
N+1 query in an ORM loop
system-designdatabasesn-plus-one - Code AnalysisHard
Cache stampede on expiry
system-designcachingstampede - Code AnalysisIntermediate
Retry storm without backoff
system-designavailabilityretry - Code AnalysisHard
Read-modify-write lost update
system-designconcurrencydatabases - Code AnalysisHard
Hot shard from a skewed key
system-designshardinghot-partition - Code AnalysisBeginner
NULL comparison bug
system-designdatabasessql - Code AnalysisIntermediate
Implicit type cast kills index
system-designdatabasesindexes - Code AnalysisIntermediate
Offset pagination instability
system-designapi-designpagination - Code AnalysisBeginner
Unbounded SELECT without index
system-designdatabasesindexesreplication - Code AnalysisIntermediate
Double-charge on retry
system-designidempotencyapi-design - Code AnalysisHard
Sync replication availability trap
system-designreplicationcap-theorem - Code AnalysisIntermediate
Consumer without dedup
system-designmessagingidempotency - Code AnalysisIntermediate
Local rate limiter drift
system-designrate-limitingdistributed - Code AnalysisBeginner
Missing index on a foreign-key join
system-designdatabasesindexes - Code AnalysisHard
Fan-out-on-write celebrity
system-designnews-feedfan-out - Code AnalysisIntermediate
Full table lock from a long transaction
system-designdatabasesconcurrency - Code AnalysisHard
Split-brain from naive failover
system-designconsensusavailability - Code AnalysisIntermediate
Unbounded in-memory queue
system-designmessagingbackpressure - Code AnalysisBeginner
Cache key without entity version
system-designcachingcache-invalidation - Code AnalysisBeginner
Thundering herd on connection pool resize
system-designdatabasesconnection-pool - Code AnalysisIntermediate
Health check that lies
system-designhealth-checks - Code AnalysisBeginner
Read replica lag on user update
system-designreplicationconsistency - Code AnalysisHard
Outbox without proper relay
system-designoutboxdistributed-transactions - Code AnalysisIntermediate
2PC coordinator down
system-designdistributed-transactions2pc - Code AnalysisHard
Thundering herd with negative result
system-designcachingcache-penetration - Code AnalysisIntermediate
Exactly-once assumption in consumer
system-designmessagingsqsidempotency - Code AnalysisHard
Unbounded cardinality metric
system-designobservabilitymetrics - Code AnalysisIntermediate
Cache eviction thrash
system-designcachingeviction - Code AnalysisHard
CAS retry livelock
system-designconcurrencydistributed-counter - Code AnalysisBeginner
Snowflake clock rollback
system-designunique-idsnowflake - Code AnalysisIntermediate
Hot key in a cache cluster
system-designcachinghot-partition - Code AnalysisIntermediate
Consistent hashing with few vnodes
system-designconsistent-hashingload-balancing - Code AnalysisHard
Retry without idempotency on a non-idempotent path
system-designidempotencyretry - Code AnalysisIntermediate
Deadlock from lock ordering
system-designdatabasesconcurrency - Code AnalysisHard
At-least-once with side effect before ack
system-designmessagingidempotency - Code AnalysisIntermediate
Read-through stampede on cold start
system-designcachingstampede - Code AnalysisHard
Fencing missing on lease-based lock
system-designconsensusleader-election - Code AnalysisIntermediate
Selective column over-fetch
system-designdatabasesperformance - Code AnalysisHard
Backpressure via unbounded broker retention
system-designmessagingbackpressure - Code AnalysisIntermediate
Negative caching with wrong TTL
system-designcachingttl - Code AnalysisBeginner
No timeout on downstream call
system-designavailabilityresilience - Code AnalysisIntermediate
Double-write to cache and DB
system-designcachingcache-invalidation
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