def merge_intervals(intervals):
    if not intervals:
        return []
    sorted_intervals = sorted(intervals, key=lambda x: x[0])
    merged = [list(sorted_intervals[0])]  # copy to avoid mutation
    for start, end in sorted_intervals[1:]:
        last_start, last_end = merged[-1]
        if start <= last_end:  # overlap or touch
            merged[-1][1] = max(last_end, end)
        else:
            merged.append([start, end])
    return merged