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代码说明：
在给定字符串中找出单词（ “单词”由小写字母字符大写字母和构成，其他非字母字符视为单词的间隔， 如、问空格号、数字等等；另外单个字母不算单词）；找到单词后，按照长度进行降序排序， （排序时如果长度相同，则按出现的顺序进行排列），然后输出到一个新的字符串中；如果某个单词重复出现多次， 则只输出一次；如果整个输入的字符串中没有找到单词，请输出空串。输出的单词之间使用一个“空格”隔开， 最后一个单词后不加空格。(Identify the word (" word" character from the uppercase and lowercase letters constituted, other non-alphabetic characters as word spacing, such as asking a space number, number, etc. another is not a single letter word) in a given string find words after descending order according to length, (if the same length, the order stated that appears when sorting arranged), and then output to a new string If a word is repeated several times, only output once If the entire input string is not found in word, outputs an empty string. Used between the output of a word " space" separated, no spaces after the last word.)
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