可以使用+操作符连接多个字符串字面值。

To concatenate multiple String literals, use the + operator.

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如果到达输入字符串的末尾，弹出所有操作符并添加到结果字符串。

If the end of the input string is found, pop all operators and append them to the result string.

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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。

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

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