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# 0253. Meeting Rooms II

<https://leetcode.com/problems/meeting-rooms-ii>

## Description

Given an array of meeting time intervals `intervals` where `intervals[i] = [starti, endi]`, return *the minimum number of conference rooms required*.

**Example 1:**

```
**Input:** intervals = [[0,30],[5,10],[15,20]]
**Output:** 2
```

**Example 2:**

```
**Input:** intervals = [[7,10],[2,4]]
**Output:** 1
```

**Constraints:**

* `1 <= intervals.length <= 104`
* `0 <= starti < endi <= 106`

## ac1: priority queue

how to initialize priority queue: `PriorityQueue<Interval> pq = new PriorityQueue<Interval>((Interval i1, Interval i2) -> {return i1.end - i2.end;});`

```java
/**
 * 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; }
 * }
 */

// min heap/priority queue
class Solution {
    public int minMeetingRooms(Interval[] intervals) {
        // edge cases
        if (intervals.length == 0) return 0;

        PriorityQueue<Interval> pq = new PriorityQueue<Interval>((Interval i1, Interval i2) -> {return i1.end - i2.end;});

        Arrays.sort(intervals, (Interval i1, Interval i2) -> {return i1.start - i2.start;});

        pq.offer(intervals[0]);

        for (int i = 1; i < intervals.length; i++) {
            Interval newM = intervals[i];
            Interval oldM = pq.poll();
            if (newM.start < oldM.end) {
                pq.offer(newM);
            } else {
                oldM.end = newM.end;
            }
            pq.offer(oldM);
        }

        return pq.size();
    }
}
/**
priority queue, sort by end time
intervals, sort by begin time
new meeting -> check queue -> if start time < end time, new room; else extend then end time.
**/
```

## ac2:

扫描线

```java
/**
 * 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) {
        // edge cases
        if (intervals == null || intervals.length == 0) return 0;

        // get start/end points
        int[] starts = new int[intervals.length];
        int[] ends = new int[intervals.length];
        for (int i = 0; i < intervals.length; i++) {
            starts[i] = intervals[i].start;
            ends[i] = intervals[i].end;
        }

        // sort for future use
        Arrays.sort(starts);
        Arrays.sort(ends);

        // iterate 
        int room = 0;
        for (int s = 0, e = 0; s < starts.length; s++) {
            if (starts[s] < ends[e]) {
                room++;
            } else {
                e++;
            }
        }

        return room;
    }
}
```