// put end into heap with comparator
	private static int minRooms(ArrayList<Interval> intervals) {
		Collections.sort(intervals, new ComparactorInterval());
		PriorityQueue<Integer> heap = new PriorityQueue<Integer>();
		int curRoom = 0;
		int maxRoom = 0;

		for (int i = 0; i < intervals.size(); i++) {
			int curStart = intervals.get(i).start;
			int curEnd = intervals.get(i).end;
			while (!heap.isEmpty() && heap.peek() <= curStart) {
				heap.poll();
				curRoom--;
			}
			heap.add(curEnd);
			curRoom++;
			maxRoom = Math.max(maxRoom, curRoom);
		}
		return maxRoom;
	}

	private static class ComparactorInterval implements Comparator<Interval> {
		public int compare(Interval a, Interval b) {
			if (a.start == b.start) {
				return a.end - b.end;
			}
			return a.start - b.start;
		}
	}