实现从键盘终端输入语法正确的算术表达式，计算出表达式的值。为了避免算符的二义性，可以假设表达式中只包含实型正常数运算符包括加、减、乘、除四种基本运算，可以包含圆括号。-Realized syntactically correct input from the keyboard terminal arithmetic expressions to calculate the value of the expression. To avoid ambiguity operator, it can be assumed only real expression contains the normal number of operators include addition, subtraction, multiplication, and division four basic operations, may contain parentheses.