说明：  要求：构建一个表达式树，当输入中缀表达式时，输出并打印其前缀及后缀表达式 实现：构造一个标准的表达式树的类，它涵盖先序、中序、后序遍历操作，通过不同顺序的打印操作来实现转换操作；相对于使用栈来进行转换，免去了中缀转成前缀时进行的逆序操作
(Requirements: to construct an expression tree, when the input infix expression, output and print the prefix and suffix expression To achieve: constructing a standard expression tree, it covers the first order, in sequence, after the traversal operation, through the printing operation sequence to achieve the conversion operation; compared with the use of the stack to convert, eliminates the inverse operation of the infix turn into a prefix)