1）使用先序遍历序列构造算法,使用二叉链表，表示算术表达式2*3+6/3的标识符树；

2）求标识符树的先序、中序、后序序列，各存入数组中并显示结果；

3) 求树中叶子结点的个数

附加：3）使用后序序列求表达式的值（使用栈实现）；

-1) Use a preorder traversal sequence construction algorithm, using the linked list of binary, indicating that the arithmetic expression 2* 3+6/3 of identifier tree

2) Find the identifier tree preorder, inorder, postorder sequence, stored in the array and display the results

3) Find the number of leaf nodes in the tree

Additional: 3) Use the order sequence to evaluate the expression (using a stack implementation)