Lintcode 919. Meeting Rooms II

Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), find the minimum number of conference rooms required.

Example

Example1

Input: intervals = [(0,30),(5,10),(15,20)]
Output: 2
Explanation:
We need two meeting rooms
room1: (0,30)
room2: (5,10),(15,20)

Example2

Input: intervals = [(2,7)]
Output: 1
Explanation: 
Only need one meeting room

Code (Java):

/**
 * Definition of Interval:
 * public classs Interval {
 *     int start, end;
 *     Interval(int start, int end) {
 *         this.start = start;
 *         this.end = end;
 *     }
 * }
 */

public class Solution {
    /**
     * @param intervals: an array of meeting time intervals
     * @return: the minimum number of conference rooms required
     */
    public int minMeetingRooms(List<Interval> intervals) {
        if (intervals == null || intervals.size() == 0) {
            return 0;
        }

        List<Event> events = new ArrayList<>();
        for (Interval interval : intervals) {
            events.add(new Event(interval.start, 0));
            events.add(new Event(interval.end, 1));
        }

        Collections.sort(events, new MyEventComparator());

        int ans = 0;
        int activeMeetings = 0;

        for (Event event : events) {
            if (event.flag == 0) {
                activeMeetings++;
            } else {
                activeMeetings--;
            }

            ans = Math.max(ans, activeMeetings);
        }

        return ans;
    }
}

class Event {
    int time;
    int flag; // 0 start, 1 end

    public Event(int time, int flag) {
        this.time = time;
        this.flag = flag;
    }
}

class MyEventComparator implements Comparator<Event> {
    @Override
    public int compare(Event a, Event b) {
        if (a.time != b.time) {
            return a.time - b.time;
        }

        return b.flag - a.flag;
    }
}