Instacart·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026

Summary

Got a coding question at Instacart that looked simple on the surface but had enough edge cases to make you second-guess yourself the whole way through.

Questions Asked (1)

Given a log of worker entry and exit events, implement a method that returns all intervals during which the worker was present in the office.

Algorithms & Data Structures

Author's notes

Spent the first few minutes just thinking about what the input actually looked like.

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AI HintsAI Generated

Suggested Approach

First, clarify the input format and edge cases. Then, sort events by timestamp and iterate through them, tracking the current state and recording intervals when the worker is present. Merge overlapping intervals if necessary.

Pro tip: Discuss how you would handle multiple workers or unsorted events, and mention that you would validate the input to ensure no negative durations or unmatched exits.

1. Clarify requirements and edge cases

Ask about the event format, whether events are sorted, if there can be multiple workers, and how to handle invalid sequences (e.g., exit without entry).

2. Choose data structures and algorithm

Decide to sort events by timestamp if not already sorted. Use a counter or boolean flag to track presence, and a list to collect intervals.

3. Iterate through events and build intervals

Process each event in order: on entry, if not already present, mark start time; on exit, if present, record interval and mark absent. Handle edge cases like consecutive entries or exits.

4. Merge overlapping intervals (if needed)

If the problem allows overlapping intervals (e.g., due to multiple entries without exits), merge them to produce non-overlapping intervals.

5. Analyze complexity and test

State time complexity (O(n log n) due to sorting) and space complexity (O(n)). Walk through test cases: normal, empty, single event, unsorted, and invalid sequences.

Key Points to Mention

Sorting events by timestamp if not already sorted
Using a counter or boolean to track presence
Handling edge cases: empty log, single event, unmatched entry/exit
Merging overlapping intervals if multiple entries occur without exits
Time and space complexity analysis
Validation of input and assumptions

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