Shopify·Software Engineer·Onsite - Coding / Algorithms·Intermediate

Intermediate

Jun 2026

Summary

Shopify software engineer coding round, got a grid robot simulator problem. Pretty clean problem statement but the follow-up with multiple robots sharing the grid is where things get genuinely interesting. Not a lot of metadata to share since this is just the problem itself, no debrief info.

Questions Asked (2)

Build a command-driven robot simulator on a 2D grid. The robot starts unplaced, accepts commands like PLACE, MOVE, LEFT, RIGHT, and REPORT, and must never fall off the grid. Parse a stream of text commands and maintain the robot's position and heading correctly.

Algorithms & Data StructuresSystem Design

Author's notes

The core problem is pretty manageable if you just track state carefully.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design a clean object-oriented solution with a Robot class encapsulating state and a command parser. Implement commands with validation to prevent invalid moves, and test thoroughly with edge cases like boundaries and unplaced robot.

Pro tip: Demonstrate production-quality thinking by discussing how you'd extend the design for new commands or grid features, and mention the importance of logging and error handling for invalid commands.

1. Clarify requirements and constraints

Ask about grid size, command format, behavior for invalid commands, and whether the robot can be placed multiple times. Confirm that the robot must ignore commands that would cause it to fall off.

2. Design the core classes and state

Create a Robot class with position (x, y), direction, and a placed flag. Define an enum for directions and methods for each command. Consider a separate parser to convert text commands into command objects.

3. Implement command handling with validation

For each command, validate preconditions (e.g., robot must be placed for MOVE, LEFT, RIGHT, REPORT). For MOVE, compute the next position and check grid boundaries before updating. Ignore invalid commands silently or log them.

4. Test with edge cases

Write unit tests covering: placing at corners, moving to boundaries, rotating through all directions, commands before placement, invalid commands, and multiple PLACE commands. Also test a sequence of commands from the problem statement.

5. Discuss extensibility and trade-offs

Mention how the design supports adding new commands (e.g., BACK, JUMP) or obstacles. Discuss trade-offs between simple if-else parsing and a command pattern, and between mutable state and immutable updates.

Key Points to Mention

Encapsulation of robot state and behavior in a class
Command pattern or parser design for extensibility
Boundary checking before movement to prevent falling off
Handling of unplaced robot and invalid commands gracefully
Use of enums for directions and rotation logic
Unit testing and edge case coverage

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

Extend the simulator to support multiple robots on the same grid, where each command is prefixed by a robot ID. Robots cannot occupy the same cell, so any MOVE or PLACE that would cause a collision is ignored. Design and document how collision detection works, including edge cases.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

This is the part that actually requires some thought.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and assumptions, then propose a data structure for robot positions and a collision detection mechanism. Walk through the command processing flow, highlighting how collisions are checked and handled, and discuss edge cases and trade-offs.

Pro tip: Emphasize that collision detection should be atomic and consistent, and consider using a position map for O(1) lookups. Mention that ignoring invalid commands is a design choice that maintains system integrity without partial state changes.

1. Clarify Requirements and Assumptions

Ask about grid size, number of robots, command format, and whether robots can be removed. Confirm that collision means two robots in the same cell at the same time.

2. Design Data Structures

Propose a map from robot ID to position and a reverse map from position to robot ID for O(1) collision checks. Discuss trade-offs of alternative structures like a 2D array.

3. Define Collision Detection Logic

For each command, compute the target position and check if it's occupied by another robot. If occupied, ignore the command; otherwise, update both maps.

4. Handle Edge Cases

Consider commands for non-existent robots, PLACE to an occupied cell, MOVE off the grid, multiple robots attempting to move to the same cell, and robots moving into each other's positions simultaneously.

5. Discuss Trade-offs and Extensions

Talk about performance, concurrency (if applicable), and how to extend to more complex collision rules (e.g., swapping positions).

Key Points to Mention

Use a position map (e.g., HashMap) for O(1) collision detection.
Collision check must be atomic: compute target, check occupancy, then update.
Edge case: multiple robots moving to the same cell in one command batch—process sequentially.
Edge case: PLACE command to an occupied cell should be ignored.
Edge case: MOVE that would cause a robot to swap positions with another robot—should be ignored if simultaneous moves are not allowed.
Trade-off: ignoring invalid commands vs. error handling; ignoring maintains state consistency.

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