[LeetCode] Employee Free Time

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:

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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:

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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, schedule0.start = 1, schedule0.end = 2, and schedule0[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.

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//put all intervals together, coz eventually we need 
//non-overlapping intervals from everyone
//Can use a PriorityQueue to sort all intervals by start
//or just use List and apply Collections.sort()
//several formats for customizing comparator
//https://www.mkyong.com/java8/java-8-lambda-comparator-example/
//after sorted, iterate the sorted list
//if (pre.end < cur.start) --> save new Interval(pre.end, cur.start)
class Solution {
    public List employeeFreeTime(List> schedule) {
        List res = new ArrayList<>();
        List times = new ArrayList<>();
        for (List list: schedule) {
            times.addAll(list);
        }
        Collections.sort(times, ((i1, i2)->i1.start-i2.start));
        Interval pre = times.get(0);
        for (int i = 1; i < times.size(); i++) {
            Interval cur = times.get(i);
            if (cur.start <= pre.end) {
                pre.end = cur.end > pre.end ? cur.end : pre.end;
            } else {
                res.add(new Interval(pre.end, cur.start));
                pre = cur;
            }    
        }
        return res;
    }
}