说明：  设计一个哈夫曼编码/译码系统，对一个文本文件中的字符进行哈夫曼编码，生成编码文件(压缩文件,后缀名.cod)； 反过来,可将一个压缩文件译码还原成一个文本文件(.txt)。
(The design of a Huffman encoding/decoding system, to a text file of characters Huffman encoding, to generate encoded file (compressed file name suffix. Cod) in turn, can restore a compressed file into a decoder text file (. txt).)