说明：  堆栈的熟悉应用 输入一个数学表达式（假定表达式输入格式合法），计算表达式结果并输出。 数学表达式由单个数字和运算符“+”、“-”、“*”、“/”、“(、) ”构成，例如 2 + 3 * ( 4 + 5 ) - 6 / 4。
(The familiar application of the stack Enter a mathematical expression (assuming that the expression input format is valid), and calculate the expression results and output. Mathematical expressions are composed of single numbers and operators "+", "-", "*", "/", and "(") ", such as 2 + 3 * (4 + 5) - 6 / 4.)