Free Windows Between Busy Intervals

Given occupied time intervals busy, return every positive-length free interval strictly between the earliest occupied start and the latest occupied end.
For this exercise, assume intervals use integer timestamps and are half-open: [start,end). Overlapping or touching occupied intervals form one occupied block. Time before the first occupied block or after the last occupied block is excluded. With fewer than two disjoint occupied blocks, return an empty array.
Return the gaps as [start,end] rows in increasing start order. The supplied schedules have already been read from their input interface; no network calls are required.

Function
freeWindows(busy: int[][]) → int[][]

Examples
Example 1
busy = [[4,7],[1,3],[9,12]]
return = [[3,4],[7,9]]
The occupied blocks are [1,3), [4,7), and [9,12). Their two internal gaps are returned.
Example 2
busy = [[1,5],[2,3],[5,8],[10,11]]
return = [[8,10]]
The nested and touching intervals combine into [1,8). The remaining gap is [8,10).
Example 3
busy = []
return = []
No occupied blocks means there are no bounded internal gaps.

Constraints
For this exercise, assume 0 <= busy.length <= 100000.
Each row has two integers with 0 <= start < end <= 10^9.
The input may be unsorted and may contain duplicate or nested intervals.

Sort the intervals by start. Walk through them while tracking the current block's end. If the next start is strictly greater than that end, you found a gap: push [curEnd, nextStart]. Otherwise extend curEnd to the max of the two ends. That max is the pitfall. Nested intervals like [2,3) inside [1,5) will shrink your end if you just assign instead of taking the max. Use strict greater-than, not greater-or-equal, because touching intervals form one block and a zero-length gap isn't allowed. Empty input and a single block both fall out naturally as an empty result. Complexity is O(n log n) for the sort, O(n) for the sweep, fine for 100000 rows. Duplicates need no special handling. If you freeze mid-assessment, StealthCoder can hand you the sweep as a hedge, but the logic is short enough to own.

Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.