If the character is an opening parenthesis, push it onto the stack.

如果字符是个开括号，把它压入堆栈。

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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。

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If the character is a closing parenthesis, pop all operators until you reach an opening parenthesis and append them to the result string.

如果字符是个闭括号(closing parenthesis)，在遇见开括号前，弹出所有操作符，然后把它们添加到结果字符串。

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