Duolingo Backend Engineer Interview Questions

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

16questions

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CodingStart here. 4 questions · ~36 min

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

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.

Algorithms

Duolingo

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System Design7 questions · ~63 min

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

Debugging Production API BottlenecksMedium

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

production systemsapi

Duolingo

SQL vs NoSQL Trade-OffsMedium

Assesses database selection skills and ability to justify trade-offs for product requirements.

Duolingo

Real-Time Updates for MillionsHard

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

system designreal-time updates

Duolingo

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Behavioral & Leadership5 questions · ~45 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 reviewsCommunicationfeedback

Duolingo

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