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Walkthrough and analysis
Trace intervals = [[1,3],[2,6],[8,10],[15,18]].
Already sorted by start.
result = [[1,3]] (first interval)
[2,6]: 2 <= 3 (overlaps). Extend: result = [[1,6]].
[8,10]: 8 > 6 (no overlap). Add: result = [[1,6],[8,10]].
[15,18]: 15 > 10 (no overlap). Add: result = [[1,6],[8,10],[15,18]].
Final: [[1,6],[8,10],[15,18]].
time for sorting. space for result.
