def merge_intervals(intervals):
    if not intervals:
        return []

    sorted_intervals = sorted([iv[:] for iv in intervals], key=lambda iv: iv[0])
    merged = [sorted_intervals[0][:]]

    for start, end in sorted_intervals[1:]:
        last_start, last_end = merged[-1]
        if start <= last_end:
            merged[-1][1] = max(last_end, end)
        else:
            merged.append([start, end])

    return merged