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.

For example,
Given [[0, 30],[5, 10],[15, 20]],
return 2.

O(n log n) Heap solution

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/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
public class Solution {
    public int minMeetingRooms(Interval[] intervals) {
        if (intervals == null || intervals.length == 0) {
            return 0;
        }
        Arrays.sort(intervals, (a,b) -> (a.start - b.start));
        PriorityQueue<Interval> pq = new PriorityQueue<Interval>((a,b) -> (a.end - b.end));
        pq.offer(intervals[0]);
        for (int i = 1; i < intervals.length; i++) {
            Interval prev = pq.poll();
            Interval cur = intervals[i];
            if (cur.start >= prev.end) {
                prev.end = cur.end;
            } else {
                pq.offer(cur);
            }
            pq.offer(prev);
        }
        return pq.size();
    }
}