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Combine overlapping
Given a list of intervals, merge all overlapping intervals.
Example: [[1,3], [2,6], [8,10], [15,18]] becomes [[1,6], [8,10], [15,18]]. The [1,3] and [2,6] overlap, so they merge to [1,6]. Before reading the solution, think: what sort order helps you merge efficiently?
Constraint: . Sorting by start time gives you time using space instead of checking all pairs.
