Merge overlapping intervals

Given an array of closed intervals `intervals`, where each interval is `[start, end]` and `start <= end`, merge all overlapping intervals and return an array of the merged, non-overlapping intervals sorted by start time. Two intervals `[a, b]` and `[c, d]` overlap if `c <= b` (assuming `a <= c`). #### Input - `intervals`: list of `n` intervals #### Output - A list of merged intervals. #### Example Input: `[[1,3],[2,6],[8,10],[15,18]]` Output: `[[1,6],[8,10],[15,18]]` #### Constraints - `1 ≤ n ≤ 10^5` - Interval endpoints fit in 32-bit integers.