253. 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.

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

/**
 * 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;
        if(intervals.length == 1) return 1;

        Comparator<Interval> comp = new Comparator<Interval>(){
            @Override
            public int compare(Interval a, Interval b){
                if(a.start == b.start) return a.end - b.end;
                return a.start - b.start;
            }
        };

        Arrays.sort(intervals, comp);
        Comparator<Interval> comp2 = new Comparator<Interval>(){
            @Override
            public int compare(Interval a, Interval b){

                return a.end - b.end;
            }
        };
        PriorityQueue<Interval> queue = new PriorityQueue<>(10, comp2);
        int count =1;
        queue.offer(intervals[0]);

        for(int i=1; i< intervals.length; i++){
            Interval top = queue.peek();
            Interval next = intervals[i];
            if(next.start >= top.end){
                queue.poll();
            }else{
                count++;
            }

            queue.offer(next);
        }

        return count;
    }
}

improve 1 : for comp2, you only need to save the ending time queue, improve 2 : you only need to measure the queue size in the end.