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

    sorted_intervals = sorted(intervals, key=lambda iv: (iv[0], iv[1]))

    merged = [[sorted_intervals[0][0], sorted_intervals[0][1]]]

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

    return merged