advanced pattern

Merge Intervals

Sort intervals and merge overlapping ones. Common in scheduling and range problems.

Time

O(n log n)

Space

O(n)

🧠Mental Model

“Organizing a shared calendar - sort meetings by start time, then merge overlapping slots into single blocks.”

Verbal cue: Sort by start, extend end if overlap.

🎯Recognition Triggers

When you see these patterns in a problem, consider this approach:

intervalsmergeoverlappingschedulemeeting roomsinsert interval

📝Code Template

Merge Intervals Template

1function mergeIntervals(intervals) {
2  if (intervals.length === 0) return [];
3  
4  // Sort by start time
5  intervals.sort((a, b) => a[0] - b[0]);
6  
7  const merged = [intervals[0]];
8  for (let i = 1; i < intervals.length; i++) {
9    const [start, end] = intervals[i];
10    const last = merged[merged.length - 1];
11    
12    if (start <= last[1]) {
13      last[1] = Math.max(last[1], end);
14    } else {
15      merged.push([start, end]);
16    }
17  }
18  
19  return merged;
20}

Complexity rationale: Dominated by sorting. Merge pass is O(n). Output can be O(n) in worst case.

💡Interview Tips

1Always clarify if intervals are open [) or closed []
2Meeting Rooms II uses a min-heap for end times
3For insert, find position then merge - or merge while inserting

⚠️Common Mistakes

✕Forgetting to sort first
✕Not using max() when extending end time
✕Off-by-one with inclusive/exclusive boundaries