Given sorted non-overlapping intervals and a new interval, insert it and merge if necessary.
Example: [[1,3],[6,9]] + [2,5] becomes [[1,5],[6,9]]. The new interval [2,5] overlaps with [1,3], merging to [1,5]. The problem tests your ability to handle three regions: before, overlapping, and after. Tricky part: multiple existing intervals can merge with the new one in a cascade.