56. 合并区间（Merge Intervals）¶

输入: intervals = [[1,3],[2,6],[8,10],[15,18]]
输出: [[1,6],[8,10],[15,18]]

public int[][] merge(int[][] intervals) {
    if (intervals.length == 0) return new int[0][];
    Arrays.sort(intervals, (a, b) -> a[0] - b[0]);
    List<int[]> res = new ArrayList<>();
    int start = intervals[0][0], end = intervals[0][1];
    for (int i = 1; i < intervals.length; i++) {
        if (intervals[i][0] <= end) {            // 重叠
            end = Math.max(end, intervals[i][1]);
        } else {
            res.add(new int[]{start, end});
            start = intervals[i][0];
            end = intervals[i][1];
        }
    }
    res.add(new int[]{start, end});
    return res.toArray(new int[0][]);
}