Merge overlapping time intervals

## Problem You are given a list of **closed** intervals `intervals`, where each interval is `[start, end]` and `start <= end`. Merge all intervals that overlap and return the resulting list of merged intervals. Two intervals `[a, b]` and `[c, d]` are considered overlapping if `c <= b` (i.e., touching at an endpoint also counts as overlapping). ## Input - `intervals`: a list of `n` intervals, each represented as two integers `[start, end]`. ## Output - A list of merged, non-overlapping intervals, sorted by increasing `start`. ## Constraints - `0 <= n <= 1e5` - `-1e9 <= start <= end <= 1e9` ## Example Input: `[[1,3],[2,6],[8,10],[15,18]]` Output: `[[1,6],[8,10],[15,18]]`