Sweep Line & Interval

思路：

不需要检测每一时刻，只需要检测起点或者终点的位置！（交点变化的位置只有起点或者终点）

将数据拆成二元组 [起点/终点标识，时刻]
按照时刻排序
依次扫描

...the simple but powerful idea of a sweep line: a vertical line that is conceptually “swept” across the plane. In practice, of course, we cannot simulate all points in time and so we consider only some discrete points.
-- TopCoder: https://www.topcoder.com/community/competitive-programming/tutorials/line-sweep-algorithms/

Template for Point

class Point{
  int time;
  int flag; // 1=start, 0=end
  Point(int time, boolean flag){
    this.time = time;
    this.flag = flag;
  }
  public static Comparator<Point> pointComparator = new Comparator<Point>() {
    public int compare(Point p1, Point p2){
      // 如果时间一致，如果两个都是start或都是end，那么它两相等。如果p1是start而p2是end，那么p1 > p2
      if (p1.time == p2.time) return p1.flag - p2.flag; 
      else return p1.time - p2.time;
    }
  }