def merge_intervals(intervals: List[List[int]]) -> List[List[int]]:
    # 注意这个排序有讲究的，要按照左端点排序
    intervals.sort(key = lambda x: x[0])
    res = []
    last_start, last_end = intervals[0][0], intervals[0][1]
    for x, y in intervals:
        if x <= last_end:
            last_end = max(last_end, y)
        else:
            res.append([last_start, last_end])
            last_start, last_end = x, y
    res.append([last_start, last_end])
    return res