Nextdoor·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

Coding round at Nextdoor for an MLE role. One problem the whole time, a calendar/scheduling variant that looked like a standard intervals question until the weekday wrapping stuff showed up.

Questions Asked (1)

Given a list of intervals where each interval has a weekday (Monday through Sunday), a start time, and an end time in HH:MM format, merge all overlapping intervals and return the merged result. Intervals can span across consecutive weekdays.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Recognized it as LC 56 pretty fast, but the weekday dimension threw me off for a minute.

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AI HintsAI Generated

Suggested Approach

Convert each interval into a linear representation (e.g., minutes since Monday 00:00), then sort by start time and merge overlapping intervals by comparing end times. Handle intervals that span across weekdays by treating the week as a continuous timeline, and consider edge cases like intervals that wrap around the week.

Pro tip: Clarify with the interviewer whether intervals that touch at endpoints (e.g., 10:00-11:00 and 11:00-12:00) should be merged, as this often trips candidates. Also, mention that you'd validate input and discuss time complexity trade-offs.

1. Clarify requirements and edge cases

Ask about interval inclusivity, whether intervals can wrap around the week (e.g., Sunday to Monday), and if the input is sorted. Confirm the output format.

2. Choose a representation

Convert each interval to a linear scale (e.g., minutes from Monday 00:00) to simplify comparisons. For intervals spanning multiple days, compute start and end as total minutes.

3. Sort and merge

Sort intervals by start time. Iterate through them, merging if the current interval's start is less than or equal to the previous merged interval's end. Update the end to the maximum of the two.

4. Handle week wrap-around

If intervals can wrap around the week, consider duplicating the week or splitting intervals that cross the week boundary. Alternatively, treat the week as circular and adjust merging logic.

5. Convert back and return

Convert merged intervals back to the original weekday and HH:MM format. Ensure the output is sorted and correctly represents any multi-day spans.

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 the merged intervals.
Handling intervals that span across multiple days by converting to a continuous timeline.
Edge cases: empty input, single interval, intervals that exactly touch, intervals that wrap around the week.
Trade-offs: in-place merging vs. creating new list; using minutes vs. seconds for granularity.
Potential follow-up: how to handle very large inputs or streaming intervals.

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