说明：  编写一程序求Huffman树。主要内容包括： 设计数据结构，求出Huffman树。 模拟Huffman树的形成过程。 给出Huffman编码。
(Procedures for the preparation of a Huffman tree. The main contents include: the design of data structure, obtained Huffman tree. Huffman simulated the formation of the tree. Huffman coding is given.)