程序功能说明： 根据ascⅡ码文件中各ascⅡ字符出现的频率情况创建Haffman树，再将各字符对应的哈夫曼编码写入文件中。同时，亦可根据对应的哈夫曼树，将哈夫曼编码文件解压成字符文件。

-Programming note: In accordance with Code asc Ⅱ document various characters asc Ⅱ frequency situation created Haffman tree, and then the characters in the corresponding Huffman write file. At the same time, also in accordance with the corresponding Huffman tree, the Huffman encoding of characters into a document file decompression.