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Shopify·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Shopify SWE interview that was basically a grid simulation problem. Clean problem statement, classic rover-on-a-plateau setup, but the edge cases around collision and boundary handling are where they're really testing you.

Questions Asked (1)

Q1

Build a command-line simulator for autonomous rovers on a rectangular grid. Each rover has a position and heading, receives a string of turn and move commands, and you need to output where each rover ends up. Rovers are processed sequentially, moves that go off-grid or into an occupied cell are silently ignored.

Algorithms & Data StructuresSystem Design
Author's notes

The base movement logic is pretty mechanical once you map out the heading rotations.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then outline a clean object-oriented design with a Grid class and Rover class. Walk through the algorithm step-by-step, emphasizing collision detection and sequential processing, and finally discuss testing and potential optimizations.

Pro tip: Mention that you would write unit tests for edge cases like rovers colliding at the same time or moving off-grid, and consider using a set for O(1) occupied cell lookups to keep the simulation efficient.

1. Clarify Requirements and Edge Cases

Ask questions to confirm grid dimensions, input format, command characters, and behavior for invalid moves. Clarify whether rovers can occupy the same cell initially and how collisions are handled.

2. Design Data Structures

Define a Grid class to track occupied cells and boundaries, and a Rover class with position, heading, and methods to turn and move. Use a set for occupied cells to enable O(1) collision checks.

3. Implement Command Processing

For each rover, parse its initial position and heading, then process each command character sequentially. For 'M', compute the next cell; if within bounds and unoccupied, update position and occupied set; otherwise ignore.

4. Handle Sequential Rovers and Output

Process rovers one by one, updating the grid's occupied cells after each rover finishes. Collect final positions and headings, then output them in the required format.

5. Test and Optimize

Write unit tests for edge cases: off-grid moves, collisions, multiple rovers, and boundary conditions. Discuss time complexity (O(N*M) for N rovers and M commands) and potential optimizations like early termination.

Key Points to Mention

  • Object-oriented design with separate Grid and Rover classes for modularity and testability.
  • Use of a set (or hash set) to track occupied cells for O(1) collision detection.
  • Sequential processing of rovers: each rover's final position affects subsequent rovers.
  • Silent ignoring of invalid moves (off-grid or collision) as per requirements.
  • Thorough testing including edge cases: grid boundaries, initial collisions, and multiple rovers.
  • Time and space complexity analysis: O(R * C) time where R is number of rovers and C is commands per rover, O(R) space for occupied cells.

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