Return list of intervals 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.
"""

# Definition for an Interval.
# class Interval:
#     def __init__(self, start: int = None, end: int = None):
#         self.start = start
#         self.end = end

class Solution:
    def employeeFreeTime(self, schedule: '[[Interval]]') -> '[Interval]':
        # Flattening the schedule
        intervals = [interval for employee in schedule for interval in employee]
        # Sorting by start of each Interval
        intervals.sort(key=lambda x: x.start)
        res, end = [], intervals[0].end
        # Checking for free time between intervals
        for i in intervals[1:]:
            if end < i.start:
                res.append(Interval(end, i.start))
            end = max(end, i.end)
        return res