对输入的前缀，后缀等表达式识别，检查，求值等

1.以字符序列的形式输入语法正确的前缀表示式并构造表达式。

2.用带括号的中缀表示式输出表达式。

3.实现对变量的赋值，变量的初值为0。

4.对算术表达式求值。

5.构造一个新的表达式。

-of input prefixes, and suffixes expression identification, inspection, for a value. a sequence of characters in the form of input grammar correct prefix said the ceremony and tectonic expression. 2. Bracketed with the sergeant said China-output expressions. 3. Implementation of variable assignment, the initial variables to 0. 4. Arithmetic expressions of value. 5. Construction of a new expression.