public int minMeetingRooms(Interval[] intervals) {
    if (intervals == null || intervals.length == 0)
        return 0;

    // Sort the intervals by start time
    Arrays.sort(intervals, new Comparator<Interval>() {
        public int compare(Interval a, Interval b) { return a.start - b.start; }
    });

    // Use a min heap to track the minimum end time of merged intervals
    PriorityQueue<Interval> heap = new PriorityQueue<Interval>(intervals.length, new Comparator<Interval>() {
        public int compare(Interval a, Interval b) { return a.end - b.end; }
    });

    // start with the first meeting, put it to a meeting room
    heap.offer(intervals[0]);

    for (int i = 1; i < intervals.length; i++) {
        // get the meeting room that finishes earliest
        Interval interval = heap.poll();

        if (intervals[i].start >= interval.end) {
            // if the current meeting starts right after
            // there's no need for a new room, merge the interval
            interval.end = intervals[i].end;
        } else {
            // otherwise, this meeting needs a new room
            heap.offer(intervals[i]);
        }

        // don't forget to put the meeting room back
        heap.offer(interval);
    }

    return heap.size();
}