def merge_intervals(intervals):
    if not intervals:
        return []
    sorted_intervals = sorted(intervals, key=lambda x: (x[0], x[1]))
    result = [list(sorted_intervals[0])]
    for start, end in sorted_intervals[1:]:
        last = result[-1]
        if start <= last[1]:
            if end > last[1]:
                last[1] = end
        else:
            result.append([start, end])
    return result