A group of friends decided to meetup on a chosen date. Everyone in the group provided his/her schedule on that date.
Finally you are given a joined schedule of all members, which lists all the time slots when at least one member is unavailable.

The schedule is a list of lists. Each element in the list is a pair of strings

[
[startTime1, endTime1],
[startTime2, endTime2],
[startTime3, endTime3]
]

Start time and end time follows the format HH:MM, where the first 2 digits denote hour and the last 2 digits denote minute, delimited by :.

Your job is to find all the potential time slots for the meetup. Find all time slots that

1. between 7:00 and 18:00
2. when all group members are available

Example 1:

Input: schedule = [["16:00", "16:30"], ["6:00", "7:30"], ["8:00", "9:20"], ["8:00", "9:00"], ["17:30", "19:20"]]
Output: [["7:30", "8:00"], ["9:20", "16:00"], ["16:30", "17:30"]]

Example 2:

Input: schedule = [["12:00", "17:30"], ["8:00", "10:00"], ["10:00", "11:30"]]
Output: [["7:00", "8:00"], ["11:30", "12:00"], ["17:30", "18:00"]]