Duolingo·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Duolingo software engineering interview that went deep into object-oriented design, specifically building out a full Animal Chess game from scratch. Pretty involved question with a lot of moving parts.

Questions Asked (1)

Design an Animal Chess game using object-oriented principles. This includes defining piece classes for all eight animals with their unique movement rules, modeling the 9x7 board with special terrain like rivers, traps, and dens, implementing movement and capture logic, building an API to initialize and manage game state, and writing test cases for board setup, a capture scenario, and a multi-move turn.

System DesignAPI & IntegrationsTechnical Trade-offs

Author's notes

This was a lot more than I expected from a single question.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the rules and scope, then design a class hierarchy for pieces with polymorphic movement validation. Model the board with terrain types and implement game state management with a clean API, ensuring testability through dependency injection and comprehensive test cases.

Pro tip: Demonstrate foresight by discussing extensibility: how to add new pieces or terrain without modifying existing code, and how to separate game logic from UI for testability.

1. Clarify Requirements and Scope

Ask questions to confirm the rules: animal ranks, movement specifics (e.g., rat swimming, tiger jumping), win conditions, and whether to include advanced features like undo. This ensures alignment before diving into design.

2. Design Class Hierarchy and Interfaces

Define an abstract Piece class with common attributes and a validateMove method. Create concrete subclasses for each animal, overriding movement rules. Use interfaces for terrain and board elements to promote extensibility.

3. Model Board and Terrain

Implement a Board class that holds a 2D grid of squares, each with a terrain type (land, river, trap, den). Terrain affects movement and capture rules, so encapsulate terrain behavior in classes or enums.

4. Implement Game Logic and API

Create a Game class that manages turns, validates moves, handles captures, and checks win conditions. Expose methods like initializeGame, movePiece, and getGameState. Use dependency injection for board and pieces to facilitate testing.

5. Write Test Cases

Write unit tests for board setup (correct piece placement), a capture scenario (valid and invalid captures), and a multi-move turn (sequence of moves leading to a win). Use mocking or stubs for isolated testing.

Key Points to Mention

Polymorphism for piece movement: each animal class implements its own move validation, avoiding conditionals.
Terrain effects: rivers restrict certain pieces, traps reduce rank, dens are win conditions.
Capture rules: rank comparison, with exceptions (rat captures elephant, trap mechanics).
API design: clear separation of concerns, immutable state where possible, and methods for initialization, moves, and state retrieval.
Testability: use dependency injection, mock board and pieces, and cover edge cases like invalid moves and win conditions.
Extensibility: how to add new pieces or terrain without modifying existing code, following open/closed principle.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.