Sweep line
Processing events (like the starts and ends of intervals) in sorted order while keeping a running state, such as how many are open.
Definition
Instead of comparing every interval with every other one, you turn each interval into a start event and an end event, sort all events by time, and walk through them once. A counter or a heap holds what is currently open. It answers questions like the maximum number of overlapping meetings in O(n log n).
In practice
Meetings 9-10, 9:30-11 and 10:30-12 give events at 9 (+1), 9:30 (+1), 10 (-1), 10:30 (+1), 11 (-1), 12 (-1); the count peaks at 2.
Learn it in context
Lessons that use this term.