If the character is an operator, pop operators until you reach an opening parenthesis, an operator of lower precedence, or a right associative symbol of equal precedence.
如果字符是个操作符，弹出(pop)操作符，直至遇见开括号(opening parenthesis)、优先级较低的操作符或者同一优先级的右结合符号。
Expressions must not end with an operator, function, comma, or opening parenthesis.
不可以用操作符、函数、逗号或开括号作为表达式结束符。
Operator: Sir, they are our opening hours.
接线员：先生，那是我们的营业时间。
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