Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

Instacart coding screen focused on extending a task manager system, which sounds simple but required thinking carefully about state management and sorted output. Pretty standard technical phone screen format.

Questions Asked (1)

Given an existing task manager, add an updateTaskPriority(task_id, delta) method and a getSortedPrioritizedTasks() method that returns tasks ordered by priority, where each task starts at priority 0.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The delta part tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Clarify requirements and constraints first, then propose a data structure that supports efficient updates and sorted retrieval. Discuss trade-offs between different approaches and justify your choice based on expected usage patterns.

Pro tip: Mention that you would consider using a balanced BST or a skip list to achieve O(log n) updates and O(n) sorted retrieval, but also discuss simpler alternatives like a hash map with periodic sorting if updates are infrequent.

1. Clarify Requirements

Ask about expected frequency of updates vs. retrievals, whether priorities can be negative, and if tasks can be deleted. This informs the choice of data structure.

2. Propose Data Structures

Suggest using a balanced BST (e.g., TreeMap) or a skip list to maintain tasks sorted by priority, allowing O(log n) updates and O(n) retrieval. Alternatively, a hash map with a dirty flag and sorting on demand.

3. Analyze Trade-offs

Compare time and space complexity of each approach. Discuss scenarios where one is better: e.g., frequent updates favor BST, infrequent updates favor lazy sorting.

4. Handle Edge Cases

Consider tasks with equal priorities (stable ordering?), negative deltas, and concurrency if applicable. Mention how your solution handles these.

5. Implement and Test

Outline the implementation details, including how to update priority and retrieve sorted tasks. Suggest unit tests for correctness and performance.

Key Points to Mention

Time complexity of update and retrieval operations
Space complexity and memory overhead
Stability of sorting for equal priorities
Concurrency and thread-safety considerations
Scalability and expected load
Trade-offs between simplicity and performance

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