哈弗曼树的建立和实现,方法选取一个权值最小和一个权值次小的结点来构成一个新的父母结点，递归建立一颗哈夫曼树-Hafu Man tree establishment and realization of the method to select a right to the minimum value and a weight value of small nodes to form a new parent node, recursive establish a Huffman tree