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21. Merge Intervals

mediumAsked at Grab

Merge overlapping intervals — Grab uses this as a sort-plus-sweep classic for scheduling problems.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array of intervals where intervals[i] = [start_i, end_i], merge all overlapping intervals and return an array of the non-overlapping intervals that cover all the intervals in the input.

Constraints

  • 1 <= intervals.length <= 10^4
  • 0 <= start_i <= end_i <= 10^4

Examples

Example 1

Input
intervals = [[1,3],[2,6],[8,10],[15,18]]
Output
[[1,6],[8,10],[15,18]]

Example 2

Input
intervals = [[1,4],[4,5]]
Output
[[1,5]]

Approaches

1. Pairwise overlap check

Repeatedly scan for any overlapping pair and merge them.

Time
O(n^2)
Space
O(n)
// while changed: scan pairs and merge
// brittle when n grows

Tradeoff:

2. Sort by start then sweep

Sort by start; sweep once, extending the current interval's end whenever the next interval overlaps.

Time
O(n log n)
Space
O(n)
function merge(intervals) {
  if (intervals.length === 0) return [];
  intervals.sort((a, b) => a[0] - b[0]);
  const out = [intervals[0]];
  for (let i = 1; i < intervals.length; i++) {
    const last = out[out.length - 1];
    if (intervals[i][0] <= last[1]) last[1] = Math.max(last[1], intervals[i][1]);
    else out.push(intervals[i]);
  }
  return out;
}

Tradeoff:

Grab-specific tips

Grab interviewers love this for ride-window scheduling — frame as collapsing overlapping driver availability blocks in a regional partition.

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