说明：  实现哈夫曼编码，要求对一个.txt文件(为测试方便，不允许有中文和全角字符出现)进行编码，生成一个只有0和1字符的编码文件。
(The realization of Huffman encoding, requirements for a.Txt file (for convenient test, does not allow Chinese and full width characters) encoding, generating a only 0 and 1 character encoding files.)