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

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

    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