LeetCode : 56. Merge Intervals 合并区间

试题
Given a collection of intervals, merge all overlapping intervals.

Example 1:

Input: [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: Since intervals [1,3] and [2,6] overlaps, merge them into [1,6].
Example 2:

Input: [[1,4],[4,5]]
Output: [[1,5]]
Explanation: Intervals [1,4] and [4,5] are considered overlapping.

代码
根据实际合并逻辑，我们首先假想所有区间都放在一条数轴上，然后我们会从前往后开始合并。所以要对区间按左边界进行升序排序。
但是还有个问题是如果左边界相等时我们应该咋办？这是我们需要按右边界降序排序。之所以降序是为了避免出现左边界相等，但右边界过小，导致合并时错分为两个区间问题。

class Solution {
    public int[][] merge(int[][] intervals) {
        if(intervals==null || intervals.length==0) return new int[0][0];
        
        Arrays.sort(intervals, (a,b) -> a[0]!=b[0]?a[0]-b[0]:b[1]-a[1] );
        ArrayList out = new ArrayList<>();
        
        int[] tmp = intervals[0];
        for(int i=1; i