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

    sorted_intervals = sorted(
        ([interval[0], interval[1]] for interval in intervals),
        key=lambda interval: interval[0],
    )

    merged = [sorted_intervals[0]]
    for start, end in sorted_intervals[1:]:
        if start <= merged[-1][1]:
            merged[-1][1] = max(merged[-1][1], end)
        else:
            merged.append([start, end])

    return merged