说明：  哈夫曼编码，实现哈夫曼树和哈夫曼编码。首先生成一个哈夫曼树，然后用这颗树对所有字符进行哈夫曼编码。
(Huffman coding, Huffman realize trees and Huffman coding. First of all, to generate a Huffman tree, and then use this tree for all characters Huffman.)