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Updated weekly · Last refresh Aug 30

Duolingo Backend Engineer Interview Questions

The questions to prepare for a Duolingo Backend Engineer interview. Questions from real interview reports rank first. Updated weekly.

17questions
~3htotal time
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1
CodingStart here. 5 questions · ~46 min
Efficient Complex Tree TraversalEasy
Practice

Traverse a deeply nested Rad Hires hierarchy iteratively and return every node's full path in preorder.

Data StructuresefficiencyDuolingo
Improving Code PerformanceMedium

Evaluates practical performance optimization skills and reasoning.

Duolingo
Testing-First Algorithm Follow-UpsMedium

Assesses ability to write correct algorithms and design high-quality test cases.

AlgorithmsDuolingo
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2
System Design7 questions · ~64 min
Data Structures for High ConcurrencyMedium

Assesses understanding of data structure trade-offs under concurrency constraints.

Trade-offsData StructuresDuolingo
Ensuring Consistency in MicroservicesMedium

Evaluates consistency strategies, trade-offs, and failure handling in distributed systems.

distributed systemsdata consistencymicroservicesDuolingo
Debugging Production API BottlenecksMedium

Evaluates troubleshooting approach using metrics, profiling, and systematic isolation.

production systemsapiDuolingo
Real-Time Updates for MillionsHard

Evaluates system design for real-time scalability and reliability at Duolingo scale.

system designreal-time updatesDuolingo
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3
Behavioral & Leadership5 questions · ~46 min
Handling Tough Code Review FeedbackMedium

Tests how you receive peer feedback in code reviews, respond constructively, and turn critique into better code and stronger team habits.

code reviewsCommunicationfeedbackDuolingo
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The finish line: interview-readyComplete all 17 questions to finish this plan.