Instacart·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026Remote

Summary

Instacart coding round that extended a task manager system with user management and a scheduled deletion feature. The scheduling constraint was the tricky part.

Questions Asked (1)

Extend a task manager class with addUser, assignTaskToUser, unassignTaskToUser, and a scheduleDeletion method that deletes a task after a given delay from a timestamp. Scheduled deletions must be processed before any other operation at the same timestamp.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The first three methods were fine, pretty much just bookkeeping with maps.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, especially the timestamp semantics and ordering guarantee. Then design the data structures and algorithms, focusing on efficient user-task mappings and a priority queue for scheduled deletions. Finally, discuss trade-offs, edge cases, and potential concurrency issues.

Pro tip: Emphasize the importance of processing scheduled deletions before other operations at the same timestamp—this often requires a tie-breaking mechanism in your priority queue. Also, consider using a lazy deletion approach to avoid expensive removals from the task list.

1. Clarify Requirements

Ask about the expected scale, timestamp granularity, and whether operations are synchronous or asynchronous. Confirm that scheduled deletions must be processed before any other operation at the same timestamp.

2. Design Data Structures

Choose appropriate structures: a map from user to set of tasks, a map from task to assigned user, and a priority queue (min-heap) for scheduled deletions keyed by timestamp with a tie-breaker to ensure deletions are processed first.

3. Implement Core Methods

Implement addUser, assignTaskToUser, and unassignTaskToUser with O(1) average time using hash maps. For scheduleDeletion, add an entry to the priority queue with the deletion timestamp and a priority flag.

4. Handle Scheduled Deletions

At each timestamp, process all scheduled deletions before any other operation. Use lazy deletion: when a deletion is due, check if the task still exists and is assigned to the same user before removing it.

5. Discuss Trade-offs and Edge Cases

Talk about time/space complexity, concurrency (if needed), and edge cases like deleting an already unassigned task or scheduling multiple deletions for the same task.

Key Points to Mention

Use a min-heap (priority queue) for scheduled deletions, ordered by timestamp and then by operation type to ensure deletions are processed first.
Maintain bidirectional mappings: user -> tasks and task -> user, to support efficient assign/unassign.
Implement lazy deletion: when a scheduled deletion is due, verify the task is still assigned to the user before removing it.
Consider thread safety if operations can occur concurrently, using locks or concurrent data structures.
Analyze time complexity: O(1) for add/assign/unassign, O(log n) for scheduling and processing deletions.
Handle edge cases: scheduling deletion for a non-existent task, multiple deletions for the same task, and unassigning before deletion.

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