n order sorted by start as given · no sort sorted by end · wrong key overlap next.start ≤ end · touching merges next.start < end · half-open extend end = max(end, next.end) end = next.end (bug) speed new set the brief's example touching [1,4],[4,5] contained [1,10],[2,3] ⏸ pause step ▸ restart ↺