Nextdoor·Machine Learning Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Nextdoor ML Engineer interview with a coding question around interval merging. Pretty standard algorithmic problem but the string parsing and day-of-week ordering added some friction I didn't fully anticipate going in.

Questions Asked (1)

Given a list of meeting time intervals as day-and-time strings in 24-hour format, merge all overlapping intervals per day and return the results sorted by day of week then start time.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The core merge logic wasn't the hard part.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

First, parse each interval string into a day of week and start/end minutes, then group intervals by day. For each day, sort intervals by start time and merge overlapping ones, and finally sort the merged intervals by day of week and start time.

Pro tip: Clarify edge cases upfront, such as intervals that touch at endpoints (e.g., 10:00-11:00 and 11:00-12:00) and whether they should be merged, and mention that you'd use a stable sort to preserve order for equal start times.

1. Parse and Group

Convert each interval string into a structured format (day, start minutes, end minutes) and group intervals by day of week.

2. Sort Within Each Day

For each day, sort the intervals by start time (and end time if needed) to prepare for merging.

3. Merge Overlapping Intervals

Iterate through the sorted intervals, merging overlapping ones by comparing the current interval's start with the previous merged interval's end.

4. Sort Final Results

Sort the merged intervals by day of week (using a predefined order) and then by start time to produce the final output.

Key Points to Mention

Time complexity: O(n log n) due to sorting, where n is the number of intervals.
Space complexity: O(n) for storing parsed intervals and merged results.
Handling edge cases: empty input, single interval, intervals that touch at endpoints, and intervals spanning midnight (if applicable).
Choice of data structures: using a dictionary to group by day and a list to store intervals per day.
Sorting stability: ensuring that intervals with the same start time are ordered consistently.
Day of week ordering: defining a custom order (e.g., Monday to Sunday) for sorting.

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