253. Meeting Rooms II

Description

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

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

Solution

Greedy, time O(n * logn), space O(n)

贪心思路，按照start对数组排序，另外用MinHeap维护现有的meeting room占用情况（几点结束）。

遍历数组，对于每个当前最早开始的meeting来说，都尝试用当前最早结束的meeting room来进行merge（这个其实很好理解，因为这个room如果不被它用，那么后面的meeting会更晚开始，没好处）。如果时间有重叠，那么给当前meeting新开一个meeting room。

/**
 * 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; }
 * }
 */
class Solution {
    public int minMeetingRooms(Interval[] intervals) {
        if (intervals == null || intervals.length == 0) {
            return 0;
        }
        // sort the intervals by start time
        Arrays.sort(intervals, (a, b) -> (a.start - b.start));
        // use a min heap to track the minimum end time of conflict intervals
        PriorityQueue queue 
            = new PriorityQueue<>((a, b) -> (a.end - b.end));
        
        for (Interval interval : intervals) {
            if (queue.isEmpty()) {
                queue.offer(interval);
                continue;
            }
             // get the meeting room that finishes earliest
            Interval earliest = queue.poll();
            if (interval.start >= earliest.end) {
                // if the current meeting starts right after 
                // there's no need for a new room, merge the interval
                earliest.end = interval.end;
            } else {
                // otherwise, this meeting needs a new room
                queue.offer(interval);
            }
            // don't forget to put the meeting room back
            queue.offer(earliest);
        }
        
        return queue.size();
    }
}

可以看出，由于intervals已经按照start排序，那么Queue中保存end即可，代码简化如下：

/**
 * 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; }
 * }
 */
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 queue = new PriorityQueue<>();   // store end times
        
        for (Interval interval : intervals) {
            if (queue.isEmpty() || interval.start < queue.peek()) {
                // if no meeting room opened or even the earliest finished meeting room won't fit
                queue.offer(interval.end);
            } else {
                // if the current meeting starts right after 
                // there's no need for a new room, merge the interval
                queue.poll();
                queue.offer(interval.end);
            }
        }
        
        return queue.size();
    }
}