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Merge overlapping weekly time intervals

NextdoorMachine Learning Engineer · Technical Screen
MediumMachine Learning EngineerTechnical ScreenCoding & Algorithms

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This question evaluates the ability to parse weekday and 24-hour time strings, reason about interval overlap, and apply efficient merging and sorting techniques for time-based intervals.

You are given a list of time intervals representing meetings. Each interval is a pair of strings in the format:

  • "<Day> <H>:<MM>" (24-hour time)
  • <Day> is one of Mon, Tue, Wed, Thu, Fri, Sat, Sun
  • Examples: "Mon 9:00", "Mon 13:00", "Tue 09:30"

Example input:

[["Mon 9:00", "Mon 13:00"], ["Mon 11:00", "Mon 16:00"]]

Two intervals overlap if they occur on the same day and their time ranges intersect (endpoints may be treated as overlapping, i.e., [9:00, 13:00] overlaps [13:00, 14:00]).

Task:

  1. Parse the strings into comparable time values.
  2. Merge all overlapping intervals (per day) and return the merged list.

Output requirements:

  • Return intervals in the same string format.
  • Sort output by day-of-week (Mon→Sun), then by start time.

Clarify/assume:

  • All intervals start < end.
  • Intervals may span different days, but each individual interval’s start and end are on the same day.
  • Input size can be large enough that an efficient solution is expected (better than (O(n^2))).