Intervals

Merge, insert, and count overlapping intervals after sorting by start time.

4 problems 1 Easy 2 Medium 1 Hard

Overview

Interval problems become tractable once you sort by start time. The key insight is comparing each interval's start with the previous interval's end to detect overlaps.

When to Use

Merge intervals
Insert interval
Meeting rooms (can attend all?)
Meeting rooms II (min rooms needed)
Non-overlapping intervals

Common Mistakes

Not sorting by start time first
Off-by-one when checking overlap: start <= end vs start < end
Forgetting to add the last interval after merging

Complexity

Time

O(n log n) — dominated by sorting

Space

O(n)

function mergeIntervals(intervals) {
  intervals.sort((a, b) => a[0] - b[0]);
  const merged = [intervals[0]];

  for (let i = 1; i < intervals.length; i++) {
    const last = merged[merged.length - 1];
    if (intervals[i][0] <= last[1]) {
      last[1] = Math.max(last[1], intervals[i][1]); // overlap: extend
    } else {
      merged.push(intervals[i]); // no overlap: new interval
    }
  }
  return merged;
}

Problems (4)

Hide completed

Problem	Difficulty	Companies	Freq	Link
Merge Intervals	Medium	FacebookAmazon +2	High
Insert Interval	Medium	LinkedInGoogle +1	High
Minimum Interval to Include Each Query	Hard	GoogleAmazon	Low
Meeting Rooms ★	Easy	FacebookAmazon +2	High	★

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