表达式求值算法：表达式求值是高级语言编译中的一个基本问题，是栈的典型应用实例。

任何一个表达式都是由运算数(operand)、运算符(operator)和界限符(delimiter)组成的。运算数既可以是常数，也可以是被说明为变量或常量的标识符；运算符可以分为算术运算符、关系运算符和逻辑运算符三类；基本界限符有左右括号和表达式结束符等。

由于某些运算符可能具有比别的运算符更高的优先级，因此表达式求值不可能严格地从左到右进行。为了正确地处理表达式，使用栈来实现正确的指令序列是一个重要的技术。

-Expression evaluation algorithms: the expression is evaluated in a high-level language compiler fundamental problem is a typical application example of the stack. Any expressions are the operands (operand), operators (operator) and boundary character (delimiter) composed. Either operand is a constant, it can be described as a variable or constant identifier　operators can be divided into arithmetic operators, relational operators, and logical operators into three categories　basic boundary operators and expressions have parentheses around terminator and so on. Because some operators than other operators may have a higher priority, so the expression can not be evaluated strictly from left to right. To properly handle expressions use a stack to achieve the correct sequence of instructions is an important technology.