以字符序列的形式从键盘输入语法正确的，不含变量的整型表达式。利用算符优先关系，实现对算术四则混合运算表达式的求值，并在求值中运算符栈、运算数栈、输入字符和主要操作的变化过程-In the form of a sequence of characters from the keyboard input syntax is correct, excluding the variable of integer expressions. Change operator precedence relati