实现二叉树的先序遍历，中序遍历和后序遍历的递归和非递归算法，以及层次遍历的算法。

用二叉树来表示代数表达式，树的每一节点包括一个运算树，代数表达式中包含“+”，“—”，“*”，“/”和一位整数且没有错误，按照先乘除后加减的原则构造二叉树。

构造一颗哈夫曼树，输出对应的哈夫曼编码和平均查找长度。-Recursive and non-recursive algorithm binary tree preorder, preorder and post-order traversal, and the level traversal algorithms. Binary Tree is represented by algebraic expressions, each node of the tree, including an operational tree algebraic expression contains " +" , " -" , " *" , " /" , and an integer without error, according to the first multiplication and division principles of construction of binary addition and subtraction. Construct a Huffman tree, the output of the corresponding Huffman coding and the average length.