题目描述：

720. 词典中最长的单词 - 力扣（LeetCode） (leetcode-cn.com)

自测用例：

["w","wo","wor","worl","world"]
["a","banana","app","appl","ap","apply","apple"]
["ab","abc"]
["a"]
["a","b"]
["b","a"]
["a","b","bc"]
["ab"]
["rac","rs","ra","on","r","otif","o","onpdu","rsf","rs","ot","oti","racy","onpd"]

Java代码：

class Solution {
    public String longestWord(String[] a) {
        Arrays.sort(a);
        String ans="";
        HashSet<String> set=new HashSet<>();
        for(String s:a){
            if(s.length()==1||set.contains(s.substring(0,s.length()-1))){
                if(ans.length()<s.length())ans=s;
                set.add(s);
            }
        }
        return ans;
    }
}