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Updated weekly · Last refresh Aug 30

Duolingo Software Engineer Interview Questions

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

50questions
~11htotal time
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1
CodingStart here. 17 questions · ~228 min
Longest Substring Without RepeatsMedium
Practice

Find the longest substring with all unique characters using a sliding window.

Hash TablesStringsTwo PointersDuolingo
Implement Binary Search AlgorithmEasy
Practice

Write a binary search function to find a target value in a sorted array.

SearchingDuolingo
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2
System Design12 questions · ~161 min
Real-Time Collaboration ArchitectureHard

Tests your ability to design low-latency systems with consistency and scalability considerations.

system designdistributed systemsarchitectureDuolingo
Scalable Notification SystemHard

Tests your system design skills for reliability, scalability, and user-facing messaging.

system designarchitectureDuolingo
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3
Execution9 questions · ~121 min
SQL vs NoSQL Trade-OffsMedium

Tests data modeling judgment and understanding of consistency and scalability.

InfrastructureTrade-offsData ModelingDuolingo
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4
Security & Infrastructure5 questions · ~67 min
Map-Based Encryption and DecryptionMedium

Tests your ability to implement deterministic encryption/decryption and reason about correctness.

encryptionDuolingo
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5
Behavioral & Leadership4 questions · ~54 min
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6
More topics3 questions · ~40 min
Working on Duolingo APIMedium

Assesses practical experience building and modifying Duolingo API functionality.

data processingapiDuolingo
Sorting Strings in SQLMedium

Assesses SQL skills for ordering strings with specific rules and edge cases.

databaseSortingDuolingo
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Hands-on SQL practiceWrite and run real queries in the editor. 3 drills · ~30 min
Compare Weekly User Activity TrendsMedium
Practice
Practice drill

Aggregate user activity by week, then use LAG to compare sessions and watch time versus the prior active week.

Window FunctionsLag/LeadDate FunctionsB Lab GlobalTapadScale
SQL for Weekly Team PerformanceMedium
Practice
Practice drill

Join Duolingo task data to team metadata and calculate weekly completion rates with date grouping and conditional aggregation.

JoinsGroup ByAggregationsDuolingoEarthcam
Second Highest Trader SalaryEasy
Practice
Practice drill

Find the second highest distinct salary from a single table using basic PostgreSQL ordering and limiting.

SubqueriesRankingSortingeClinicalWorksInformaticaSpectraforce Technologies
The finish line: interview-readyComplete all 50 questions plus 3 hands-on drills to finish this plan.