11组表达式求值

代码说明：

  设计一个表达式求值的程序。该程序必须可以接受包含（，），+，-，*，/的中缀表达式，并求出结果。如果表达式正确，则输出表达式的结果；如果表达式非法，则不输出或输出其他字符。(Design an expression evaluation procedures. The program must accept (,), including +, -, *, / the infix expression, and the results obtained. If the expression is correct, the output of the expression is output; if the expression is illegal, no other characters are output or exported.)