#849 · #759 · Employee Free Time(员工空闲时间)

You are given the working schedules of several employees as a list of lists of intervals schedule[i], each sorted and disjoint. Return the list of finite intervals representing the common free time for all employees, sorted by start. Constraints: 1 <= schedule.length <= 50, 0 <= schedule[i].length <= 50, 0 <= start < end <= 10^8.