说明：  用优先队列实现霍夫曼编码，是对贪心算法实现的一个详细代码。输入文件是待编码文件，输出文件是各个字符编码信息。
(Priority queue with the realization of Huffman code is a greedy algorithm to achieve a detailed code. Input file is to be encoded file, output file is all character encoding information.)