Given an array of intervals where intervals[i] = [start_i, end_i], merge all overlapping intervals and return an array of the non-overlapping intervals that cover all the intervals in the input.
Input: intervals = [[1,3],[2,6],[8,10],[15,18]] Output: [[1, 6], [8, 10], [15, 18]] Explanation: Intervals [1,3] and [2,6] overlap, so they merge into [1,6].
Input: intervals = [[1,4],[4,5]] Output: [[1, 5]] Explanation: Intervals [1,4] and [4,5] touch at 4 and are considered overlapping.
Input: intervals = [[1,4],[2,3]] Output: [[1, 4]] Explanation: [2,3] is fully contained within [1,4].
1 <= intervals.length <= 10^4intervals[i].length == 20 <= start_i <= end_i <= 10^4intervals = [[1,3],[2,6],[8,10],[15,18]]