Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.
Example 1:
Input: intervals = [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: Since intervals [1,3] and [2,6] overlap, merge them into [1,6].
Example 2:
Input: intervals = [[1,4],[4,5]]
Output: [[1,5]]
Explanation: Intervals [1,4] and [4,5] are considered overlapping.
Constraints:
1 <= intervals.length <= 104intervals[i].length == 20 <= starti <= endi <= 104給定一個 2D矩陣 intervals，
其中每個 intervals[i] = [$start_i, end_i]$ 代表一個區間
當兩個區間有重疊時， 可以合併位一個範圍較大的區間
要求寫一個演算法來把 intervals 內中所有可以合併的區間都做合併
假設 intervals[i] = [$start_i, end_i$]
intervals[j] = [$start_j, end_j$]
if $start_i$ ≤ $start_j$ 代表 overlapStart