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:]:
        if start <= merged[-1][1]:
            if end > merged[-1][1]:
                merged[-1][1] = end
        else:
            merged.append([start, end])
    return merged