Trace intervals = [[0,30],[5,10],[15,20]].
Events: [(0,start),(5,start),(10,end),(15,start),(20,end),(30,end)].
Sorted by time (ends before starts if same time): [(0,+1),(5,+1),(10,-1),(15,+1),(20,-1),(30,-1)].
Walk through:
- Time 0: rooms = 1.
- Time 5: rooms = 2. (max so far = 2)
- Time 10: rooms = 1.
- Time 15: rooms = 2. (max still 2)
- Time 20: rooms = 1.
- Time 30: rooms = 0.
Answer: rooms.
time for sorting. space for events.