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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.
Example 1
Input: schedule = [[[1,2],[5,6]],[[1,3]],[[4,10]]]
Output: [[3,4]]
Explanation: Merging everyone's working hours gives [1,3] (from [1,2] and [1,3]) and [4,10] (which covers [5,6]). The only gap when nobody is working is [3,4].
Example 2
Input: schedule = [[[1,3],[6,7]],[[2,4]],[[2,5],[9,12]]]
Output: [[5,6],[7,9]]
Explanation: Working hours [1,3], [2,4] and [2,5] merge into [1,5]. Then [6,7] and [9,12] follow. The gaps between these blocks are [5,6] and [7,9].
Example 3
Input: schedule = [[[1,2]],[[3,4]]]
Output: [[2,3]]
Explanation: One employee works [1,2] and the other works [3,4]. Both are free between 2 and 3.
Flatten all intervals into one list, sort by start, merge overlapping ones, then gaps between merged intervals are the free time.
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