391. Number of Airplanes in the Sky

391. Number of Airplanes in the Sky
Given an interval list which are flying and landing time of the flight. How many airplanes are on the sky at most?

Example
For interval list

[
  (1,10),
  (2,3),
  (5,8),
  (4,7)
]
Return 3

Notice
If landing and flying happens at the same time, we consider landing should happen at first.

/**
 * Definition of Interval:
 * public classs Interval {
 *     int start, end;
 *     Interval(int start, int end) {
 *         this.start = start;
 *         this.end = end;
 *     }
 * }
 */


class Point{
    int time;
    int flag;
    Point(int t, int s){
      this.time = t;
      this.flag = s;
    }
    public static Comparator<Point> PointComparator  = new Comparator<Point>(){
      public int compare(Point p1, Point p2){
        if(p1.time == p2.time) return p1.flag - p2.flag;
        else return p1.time - p2.time;
      }
    };
}

public class Solution {
    /**
     * @param airplanes: An interval array
     * @return: Count of airplanes are in the sky.
     */
    public int countOfAirplanes(List<Interval> airplanes) {
        if(airplanes == null) return 0;
        
        List<Point> pointlist = new ArrayList<>();
        for(Interval intervall : airplanes) {
            pointlist.add(new Point(intervall.start, 1));
            pointlist.add(new Point(intervall.end, -1));
        }
        Collections.sort(pointlist,Point.PointComparator );
        int res = 0, cnt = 0;
        for(Point p : pointlist) {
            if(p.flag == 1) {
                cnt++;
            }
            else {
                cnt--;
            }
            res = Math.max(res, cnt);
        }
        return res;
    }
}