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class Solution:
    def longestWord(self, words: List[str]) -> str:
        """
        :type words: List[str]
        :rtype: str
        """
        words.sort()       
        save = set()        #用list存储的话，后面搜索会比较慢
        res = ""
        for word in words:
            if word[:-1]  in save or word[:-1] == '':   #单字母的单词 word[:-1] == ''
                if len(word) > len(res):
                    res = word
                save.add(word)                    
        return res