# Software Engineer
**Role:** Software Engineer · **Level:** Software Engineer II
**Company:** [Duolingo](https://scaleengineer.com/companies/duolingo)
**Difficulty:** Medium to Hard
**Salary:** US$110000 - US$140000
**Experience:** 2 - 5
**Timeline:** ~10 days
This interview process is designed to assess candidates for a Software Engineer II position at Duolingo. It evaluates technical skills, problem-solving abilities, cultural fit, and experience relevant to building and scaling educational technology products.
Canonical: https://scaleengineer.com/interviews/duolingo/software-engineer-ii-software-engineer
---
## Overall evaluation

- Technical Proficiency
- Behavioral and Cultural Fit

## Questions asked

- Tell me about a challenging project you worked on and how you overcame obstacles.
- How would you design a system to track user progress in Duolingo?
- What are your favorite programming languages and why?
- Describe a time you had to debug a complex issue.
- How do you approach learning new technologies?
- What interests you about working at Duolingo?
- If you could improve one aspect of the Duolingo app, what would it be and why?
- Explain the concept of recursion with an example.
- How would you optimize a database query?
- Describe your experience with cloud platforms like AWS or GCP.

## Preparation tips

### lists

- Review fundamental data structures and algorithms (arrays, linked lists, trees, graphs, hash maps, sorting, searching).
- Practice coding problems on platforms like LeetCode, HackerRank, or Coderbyte, focusing on medium-difficulty problems.
- Understand common software design patterns and principles (SOLID, DRY, KISS).
- Brush up on system design concepts, including scalability, reliability, and distributed systems.
- Prepare to discuss your past projects, highlighting your contributions, challenges, and learnings.
- Research Duolingo's technology stack and product offerings.
- Practice behavioral questions using the STAR method (Situation, Task, Action, Result).

### studyPlan

- {"title":"Data Structures and Algorithms Fundamentals","longDescription":"Weeks 1-2: Focus on core data structures (arrays, linked lists, stacks, queues, trees, graphs, hash tables) and their common operations and time complexities. Practice implementing them and solving problems related to them. Review basic algorithms like sorting (quicksort, mergesort) and searching (binary search).","shortDescription":"Weeks 1-2: Data Structures & Basic Algorithms. Practice implementations and problem-solving."}
- {"title":"Advanced Algorithms and System Design Concepts","longDescription":"Weeks 3-4: Dive into more advanced algorithms (dynamic programming, greedy algorithms, graph traversal - BFS/DFS). Work on medium-level LeetCode problems that combine data structures and algorithms. Start exploring system design concepts like load balancing, caching, databases, and API design.","shortDescription":"Weeks 3-4: Advanced Algorithms & Intro to System Design. Focus on medium LeetCode and system design basics."}
- {"title":"System Design and Behavioral Preparation","longDescription":"Weeks 5-6: Deepen system design knowledge. Practice designing common systems (e.g., URL shortener, Twitter feed, chat system). Prepare behavioral questions by reflecting on past experiences and structuring answers using the STAR method. Research Duolingo's product and company values.","shortDescription":"Weeks 5-6: System Design Practice & Behavioral Prep. Mock interviews and company research."}

## Location differences

- {"location":"Global","differences":{"tips":["Familiarize yourself with Duolingo's product and mission.","Be prepared to discuss your past projects in detail, focusing on your contributions and the impact.","Practice coding problems, especially those involving algorithms and data structures.","Research common cloud platforms (AWS, GCP, Azure) if relevant to the role.","Prepare questions to ask the interviewer about the team, technology stack, and company culture."],"interviewFocus":["Understanding of core computer science principles and data structures.","Ability to design scalable and maintainable software solutions.","Experience with web development frameworks and cloud technologies.","Problem-solving and debugging skills.","Communication and collaboration skills."],"commonQuestions":["How would you approach building a feature for Duolingo that personalizes learning paths based on user performance?","Describe a time you had to optimize a slow-performing piece of code. What was the issue and how did you resolve it?","How do you ensure the quality and reliability of your code in a large-scale application?","What are your thoughts on A/B testing new features for a product like Duolingo?","Tell me about a challenging technical problem you solved that involved distributed systems."]}}
- {"location":"Pittsburgh, USA","differences":{"tips":["Understand the specific challenges of building educational software at scale.","Be ready to discuss your experience with agile methodologies.","Prepare examples that showcase your ability to work cross-functionally with product managers and designers.","Think about how you would contribute to Duolingo's learning science initiatives.","Highlight any experience with internationalization and localization if applicable."],"interviewFocus":["System design and architecture skills.","Experience with data modeling and database management.","Understanding of user experience and product development cycles.","Leadership potential and team collaboration.","Adaptability to new technologies and challenges."],"commonQuestions":["How would you design a system to handle real-time feedback for language learners on Duolingo?","Describe a situation where you disagreed with a technical decision made by your team. How did you handle it?","What are the trade-offs between different database technologies for storing user progress data?","How do you stay updated with the latest trends in software development and education technology?","Tell me about a time you had to mentor a junior engineer. What was your approach?"]}}

## Round 1: Technical Coding Round 1
**Type:** Data Structures and Algorithms Interview · **Difficulty:** Medium · **Duration:** 45 min
Coding challenge focusing on data structures and algorithms.
This round focuses on your fundamental computer science knowledge. You will be given one or two coding problems that require you to implement algorithms and use appropriate data structures. The interviewer will assess your ability to write clean, efficient, and correct code, as well as your understanding of time and space complexity.
**Interviewers look for:** A structured approach to problem-solving.; Clean, efficient, and correct code.; Ability to explain trade-offs of different solutions.; Understanding of Big O notation.
**Evaluation criteria:** Problem-solving skills; Algorithmic knowledge; Coding proficiency; Understanding of time and space complexity
**Common rejection reasons:** Inability to articulate thought process clearly.; Incorrect or inefficient algorithmic solutions.; Poorly written or unreadable code.; Lack of understanding of fundamental data structures.; Failure to consider edge cases or constraints.
## Questions

- Given an array of integers, find the contiguous subarray with the largest sum.
- Implement a function to reverse a linked list.
- Find the kth smallest element in a binary search tree.

## Preparation tips

- Practice coding problems on platforms like LeetCode.
- Review common data structures and algorithms.
- Be prepared to explain your thought process step-by-step.
- Write code on a whiteboard or shared editor, focusing on readability.

## Round 2: System Design Round
**Type:** System Design Interview · **Difficulty:** Hard · **Duration:** 60 min
Design a scalable software system.
This round assesses your ability to design scalable and reliable software systems. You'll be presented with a high-level problem (e.g., design a URL shortener, design a social media feed) and expected to discuss the architecture, components, data models, APIs, and trade-offs involved. Focus on scalability, availability, and maintainability.
**Interviewers look for:** A structured approach to system design.; Ability to break down complex problems.; Consideration of various components (databases, APIs, caching, load balancing).; Understanding of trade-offs and justifications for design choices.; Clear communication of the design.
**Evaluation criteria:** System design capabilities; Scalability and performance considerations; Trade-off analysis; Understanding of distributed systems; Communication of technical design
**Common rejection reasons:** Inability to design a scalable and robust system.; Lack of consideration for edge cases and failure scenarios.; Poor understanding of trade-offs between different design choices.; Difficulty in communicating design ideas.; Not addressing non-functional requirements like scalability, reliability, and maintainability.
## Questions

- Design a system like Twitter's news feed.
- Design a URL shortening service.
- Design a real-time notification system for a web application.

## Preparation tips

- Study common system design patterns and architectures.
- Practice designing various systems.
- Understand concepts like load balancing, caching, databases (SQL vs. NoSQL), message queues, and microservices.
- Be prepared to justify your design choices.

## Round 3: Behavioral and Managerial Round
**Type:** Behavioral Interview · **Difficulty:** Medium · **Duration:** 45 min
Assessing behavioral competencies and cultural fit.
This round focuses on your behavioral and cultural fit. You'll be asked questions about your past experiences, how you handle challenges, work in teams, and your motivations. Use the STAR method (Situation, Task, Action, Result) to provide clear and concise answers.
**Interviewers look for:** Examples of collaboration and teamwork.; How you handle conflict or disagreements.; Your approach to learning and taking feedback.; Alignment with Duolingo's mission and values.; Enthusiasm for the role and company.
**Evaluation criteria:** Teamwork and collaboration; Communication skills; Problem-solving approach in past experiences; Motivation and cultural fit; Adaptability and learning agility
**Common rejection reasons:** Lack of alignment with company values.; Poor communication or interpersonal skills.; Inability to provide specific examples for behavioral questions.; Negative attitude or lack of enthusiasm.; Unrealistic salary expectations or lack of interest in the role.
## Questions

- Tell me about a time you had a conflict with a teammate and how you resolved it.
- Describe a situation where you failed. What did you learn from it?
- Why are you interested in working at Duolingo?
- How do you prioritize your work when you have multiple competing deadlines?

## Preparation tips

- Prepare examples for common behavioral questions (teamwork, conflict resolution, leadership, failure).
- Use the STAR method to structure your answers.
- Research Duolingo's mission, values, and culture.
- Be ready to discuss your career goals and why you're interested in this role.