759. Employee Free Time
Description
Intuition
其实这道题用merge interval做很方便，有个小技巧，最后sort完starts和ends array后，可以用
for (int i = 1; i < starts.length; i++) {
final int start = starts[i], preEnd = ends[i - 1];
if (preEnd < start) {
result.add(new Interval(preEnd, start));
}
}