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Interview questions from top companies, mapped to the exact round they show up in. Prep for what you will actually face.

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Online AssessmentDSA / CodingMachine CodingLow-Level Design

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The Loop aggregates publicly shared interview experiences and curated commonly-asked questions, each labeled with its source. Not affiliated with any company listed.

CC BY-NC 4.0 · © 2026

← Duolingo

Senior Backend at Duolingo

BackendSeniorRejected◆ Commonly asked· 2025

Senior backend interview. Coding-heavy early, design and behavioral later.

The loop · 6 rounds

01

Round 01 · 3 questions

DSA / Coding

Data structures & algorithms on a whiteboard or shared editor.

  • Longest consecutive sequence.
    SeniorMedium◆ Commonly asked
    #hashmap#array
    PracticeLeetCode↗GeeksforGeeks↗
  • Implement a trie (prefix tree).
    SeniorMedium◆ Commonly asked
    #trie
    PracticeLeetCode↗GeeksforGeeks↗
  • Maximum depth of a binary tree.
    SeniorEasy◆ Commonly asked
    #tree#dfs
    PracticeLeetCode↗GeeksforGeeks↗
02

Round 02 · 2 questions

Machine Coding

Build a working component or service under time pressure.

  • Build a REST endpoint with pagination, filtering and sorting.
    SeniorMedium◆ Commonly asked
    #api#pagination
    PracticeGeeksforGeeks↗Search↗
  • Implement a JSON config loader with env overrides.
    SeniorEasy◆ Commonly asked
    #config
    PracticeGeeksforGeeks↗Search↗
03

Round 03 · 2 questions

System Design

High-level architecture of a large-scale system.

  • Design a typeahead / autocomplete service.
    SeniorHard◆ Commonly asked
    #trie#cache#ranking
    PracticeGeeksforGeeks↗Search↗
  • Design a file-sharing service with permissions.
    SeniorMedium◆ Commonly asked
    #storage#acl
    PracticeGeeksforGeeks↗Search↗
04

Round 04 · 3 questions

Tech Deep-Dive

Deep questions on your core stack and fundamentals.

  • Compare rate-limiting algorithms and their trade-offs.
    SeniorMedium◆ Commonly asked
    #rate-limit
    PracticeGeeksforGeeks↗Search↗
  • Explain how you handle concurrency / race conditions.
    SeniorHard◆ Commonly asked
    #concurrency
    PracticeGeeksforGeeks↗Search↗
  • Explain connection pooling and why it matters.
    SeniorMedium◆ Commonly asked
    #databases
    PracticeGeeksforGeeks↗Search↗
05

Round 05 · 1 question

Hiring Manager

Scope, ownership, and role-fit discussion with the manager.

  • How do you run an incident and the postmortem after?
    Senior◆ Commonly asked
    #reliability#process
06

Round 06 · 2 questions

Behavioral / HR

Values, collaboration, and past-experience (STAR) questions.

  • Describe handling a tight deadline with competing priorities.
    Senior◆ Commonly asked
    #prioritization#star
  • Describe a time you improved a process or tool for your team.
    Senior◆ Commonly asked
    #ownership#star

Practice each round across every company

  • DSA / Coding
  • Machine Coding
  • System Design
  • Tech Deep-Dive
  • Hiring Manager
  • Behavioral / HR

◆ This experience is composed from commonly reported questions for this role and level. Real, individually-shared interviews will be layered in over time.