Sweep-Line

LeetCode 253 note: a heap of end times for running meetings; half-open intervals need <=

LeetCode 759 note: k-way merge all work intervals by start; any gap after the latest end so far is common free time

LeetCode 986 note: two pointers each hold an interval, the intersection is [max(starts), min(ends)], and the interval that ends first cannot meet anything else, so its pointer advances