lintcode-medium-Number of Airplanes in the Sky

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

class Solution {
    /**
     * @param intervals: An interval array
     * @return: Count of airplanes are in the sky.
     */
    public int countOfAirplanes(List<Interval> airplanes) { 
        // write your code here
        
        if(airplanes == null || airplanes.size() == 0)
            return 0;
        
        ArrayList<point> list = new ArrayList<point>();
        
        for(Interval interval: airplanes){
            list.add(new point(interval.start, 1));
            list.add(new point(interval.end, 0));
        }
        
        Collections.sort(list, 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;
                }
            } 
        });
        
        int count = 0;
        int ans = 0;
        
        for(point p: list){
            if(p.flag == 1)
                count++;
            else
                count--;
            
            ans = Math.max(ans, count);
        }
        
        return ans;
    }
    
    class point{
        int time;
        int flag;
        
        public point(int time, int flag){
            this.time = time;
            this.flag = flag;
        }
    }
    
}