def merge_intervals(intervals):
    if not intervals:
        return []
    
    # Create a copy to avoid mutating the input
    sorted_intervals = sorted(intervals, key=lambda x: x[0])
    
    merged = [list(sorted_intervals[0])]
    
    for current in sorted_intervals[1:]:
        last = merged[-1]
        
        # Check if current overlaps or touches the last merged interval
        if current[0] <= last[1]:
            # Merge them: extend the end if necessary
            last[1] = max(last[1], current[1])
        else:
            # No overlap, add as new interval
            merged.append(list(current))
    
    return merged