说明：  编程计算磁盘上（或某一目录下）所有文件的平均长度。 编程计算磁盘上（或某一目录及其各级子目录）每目录的平均文件个数。
(programming calculated on the disk (or a directory) all the average length of documents. Programming calculated on the disk (or levels of a directory and its subdirectories), the average per-directory document number.)