We are given a list schedule of employees, which represents the working time for each employee.

Each employee has a list of non-overlapping Intervals, and these intervals are in sorted order.

Return the list of finite intervals representing common, positive-length free time for all employees, also in sorted order.

Example 
 
1:
Input: schedule = [[[1,2],[5,6]],[[1,3]],[[4,10]]]
Output: [[3,4]]
Explanation:
There are a total of three employees, and all common
free time intervals would be [-inf, 1], [3, 4], [10, inf].
We discard any intervals that contain inf as they aren't finite.
Example 2:
Input: schedule = [[[1,3],[6,7]],[[2,4]],[[2,5],[9,12]]]
Output: [[
 
5,6],[7,9]]
(Even though we are representing Intervals in the form [x, y], the objects inside are Intervals, not lists or arrays. For example, schedule[0][0].start = 1, schedule[0][0].end = 2, and schedule[0][0][0] is not defined.)

Also, we wouldn't include intervals like [5, 5] in our answer, as they have zero length.

Note:

schedule and schedule[i] are lists with lengths in range [
 
1, 50].
0 <= schedule[i].start < schedule[i].end <= 10^8.






https://www.youtube.com/watch?v=VTgF52uGK0Y



/**
 * 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 List<Interval> employeeFreeTime(List<List<Interval>> schedule) {
        // merge intervels and get the empty intervals 
        List<Interval> res = new ArrayList<>();
        List<Interval> time = new ArrayList<>();
        for(List<Interval> list : schedule){
            for(Interval interval : list){
                time.add(interval);
            }
        }
        
        Collections.sort(time, (a, b) -> a.start - b.start);
        int end = time.get(0).end;
        for(Interval interval : time){
            if(interval.start > end){
                res.add(new Interval(end, interval.start)); // new Interval(end, interval.start), not new Interval<>()
            }
            end = Math.max(end, interval.end);
        }
        return res;    
    }
}