25. Meeting Rooms II

mediumAsked at Doordash

Find the minimum number of concurrent resources to cover all intervals — Doordash applies this exact pattern to Dasher shift scheduling: how many active Dashers are needed at peak overlap?

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-26

Problem

Given an array of meeting time intervals intervals where intervals[i] = [start_i, end_i], return the minimum number of conference rooms required to hold all meetings without conflict.

Constraints

1 <= intervals.length <= 10^4
0 <= start_i < end_i <= 10^6
Meetings may share start or end times

Examples

Example 1

Input

intervals = [[0,30],[5,10],[15,20]]

Output

Explanation: [0,30] overlaps with [5,10] (2 rooms needed); [15,20] reuses the room freed at 10.

Example 2

Input

intervals = [[7,10],[2,4]]

Output

Explanation: No overlap; one room suffices.

Approaches

1. Chronological event sweep

Create +1 events for starts and -1 events for ends; sort all events by time; track running count and record the peak.

Time: O(n log n)
Space: O(n)

function minMeetingRooms(intervals) {
  const events = [];
  for (const [s, e] of intervals) {
    events.push([s, 1]);
    events.push([e, -1]);
  }
  events.sort((a, b) => a[0] - b[0] || a[1] - b[1]);
  let rooms = 0, maxRooms = 0;
  for (const [, delta] of events) {
    rooms += delta;
    maxRooms = Math.max(maxRooms, rooms);
  }
  return maxRooms;
}

Tradeoff:

2. Min-heap of end times

Sort by start; use a min-heap of end times. For each new meeting, reuse a room if its end <= current start; otherwise allocate a new room.

Time: O(n log n)
Space: O(n)

function minMeetingRooms(intervals) {
  if (!intervals.length) return 0;
  intervals.sort((a, b) => a[0] - b[0]);
  const ends = [];
  for (const [start, end] of intervals) {
    if (ends.length && ends[0] <= start) {
      ends.shift();
    }
    ends.push(end);
    ends.sort((a, b) => a - b);
  }
  return ends.length;
}

Tradeoff:

Doordash-specific tips

Doordash frames this as: 'Given Dasher shift windows, what's the minimum fleet size for peak coverage?' They expect you to recognise both approaches (sweep-line vs heap) and justify your choice — the heap approach is often cleaner to explain for the scheduling metaphor. Watch for the tie-breaking edge: when a meeting ends exactly as another starts, should you free the room first? (Yes — mention this explicitly.) Follow-ups typically go to online streaming versions or weighted intervals.

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Output

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