class EmployeeInterval:

    def __init__(self, interval, employeeIndex, intervalIndex):
        self.interval = interval  # interval representing employee's working hours
        # index of the list containing working hours of this employee
        self.employeeIndex = employeeIndex
        self.intervalIndex = intervalIndex  # index of the interval in the employee list

    def __lt__(self, other):
        # min heap based on meeting.end
        return self.interval.end < other.interval.end

class Solution:
  def findEmployeeFreeTime(self, schedule):
      result = []
      heap = []
      schedule = [
        [
            EmployeeInterval(employee_int, i, j) for j, employee_int in enumerate(employee_sch)
        ] for i, employee_sch in enumerate(schedule)
      ]

      for employee_sch in schedule:
        heappush(heap, employee_sch[0])

      while heap:
        min_interval = heappop(heap)
        if min_interval.intervalIndex < len(schedule[min_interval.employeeIndex]) - 1:
            heappush(heap, schedule[min_interval.employeeIndex][min_interval.intervalIndex+1])

        if len(heap) > 0 and min_interval.interval.end < heap[0].interval.start:
            result.append([min_interval.interval.end, heap[0].interval.start])
        
      return result