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