Meeting Rooms 2.java

import java.util.Arrays;
import java.util.Comparator;


public class Solution {

    //A method to return the number of used rooms
    public int minMeetingRooms(int[][] intervals) {

        //If input == null, does not process it
        if (intervals.length == 0) {
            return 0;
        }

        //defining the start and end arrays
        Integer[] start = new Integer[intervals.length];
        Integer[] end = new Integer[intervals.length];

        //Assigning values from the intervals array
        //to the start and end array
        for (int i = 0; i < intervals.length; i++) {
            start[i] = intervals[i][0];
            end[i] = intervals[i][1];
        }

        //Sort the start and end array in increasing order
        Arrays.sort(
                 end,
                new Comparator<Integer>() {
                    public int compare(Integer a, Integer b) {
                        return a - b;
                    }
                });

        Arrays.sort(
                start,
                new Comparator<Integer>() {
                    public int compare(Integer a, Integer b) {
                        return a - b;
                    }
                });


        //Pointers to loop over the start and end array
        int startPointer = 0, endPointer = 0;

        //The number of rooms
        int usedRooms = 0;

        //Loops over the arrays until it checks
        //every meeting in the intervals array
        while (startPointer < intervals.length) {

            //If this is true, we have an empty room and
            //do not require a new room for the meeting
            if (start[startPointer] >= end[endPointer]) {
                usedRooms -= 1;
                endPointer += 1;
            }

            //Otherwise, we make an empty room to hold the meeting.
            usedRooms += 1;
            startPointer += 1;

        }

        //Returns the number of rooms used
        return usedRooms;
    }

}