# Merge Overlapping Intervals with a Sweep Implement `merge_intervals(intervals)` for closed integer intervals `[start, end]` with `start <= end`. Return their union as non-overlapping intervals sorted by start. Intervals sharing an endpoint merge. Constraints: up to `200000` intervals; every endpoint is an integer in `[-10^9, 10^9]`. Return an array of two-integer arrays. Do not mutate the input. Aim for `O(n log n)` time. ```hint Focus on boundary policy Test nested intervals, disjoint intervals, duplicate intervals, and two intervals that share one endpoint. ```