Intervals

1. 15.1What Overlap MeansTwo intervals overlap when each starts before the other ends. Decide whether touching endpoints count.10 min
2. 15.2Sort by Start, Then MergeAfter sorting by start, overlapping intervals are neighbours, so one pass merges them.12 min
3. 15.3Sort by End for Greedy ChoicesTo keep the most non-overlapping intervals, always pick the one that ends earliest: it leaves the most room for the rest.12 min
4. 15.4Sweep Line: Counting What's ActiveTurn each interval into a +1 event at its start and a −1 at its end, sort events, and the running sum is how many are active.12 min

In order of difficulty. Each one shows the pattern it teaches.

1. 15.1Merge IntervalsMediumMerge Intervals

   The core interval technique: sort by start and extend the last block while intervals overlap.

2. 15.2Insert IntervalMediumMerge Intervals

   When the list is already sorted and disjoint, one linear pass does it: copy before, merge the overlap, copy after.

3. 15.3Meeting RoomsEasyMerge Intervals

   Sort by start and check neighbours: any overlap means one person can't attend everything.

4. 15.4Meeting Rooms IIMediumMerge Intervals

   The maximum number of simultaneous intervals, with a min-heap of end times (or a sweep line).

5. 15.5Non-overlapping IntervalsMediumGreedy Choice

   Activity selection: sort by end, keep every interval compatible with the last kept, and remove the rest.

6. 15.6Minimum Number of Arrows to Burst BalloonsMediumGreedy Choice

   The same greedy from the other side: shoot at the earliest end, and that arrow bursts every balloon that started before it.

7. 15.7Interval List IntersectionsMediumTwo Pointers: Read and Write

   Two sorted interval lists walked with two pointers: the intersection is [max of starts, min of ends], then advance the one that ends first.