Intervals

Sort then scan, intervals sorted by right end, left and right passes, pairing and parity invariants, adjacent differences, and “remove what exceeds the average”, each with why greedy is correct

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

Sliding window over meetings; track total duration in a k-sized window and compute free span between boundaries.

LeetCode 2402 note: one heap for free rooms and one for busy rooms; a delayed meeting keeps its original length