说明：  利用哈夫曼编码实现对一个文本文件的内容加密，假设该文本文件只能包含小写字母、空格、逗号、和句号等字符。该程序是实现对文件中的字符进行编码、输出各个字符对应的编码、输出文件的哈夫曼编码。
(The use of Huffman coding to achieve the contents of a text file encryption, the assumption that the text file can contain only lowercase letters, spaces, commas, and characters such as the full stop. The program is to achieve the document s character encoding, the output corresponding to the various character encoding, the Huffman coding output file.)