非常巧妙的方法：

class Solution {
public:
    string longestWord(vector<string>& words) {
        unordered_set<string> hash;
        sort(words.begin(),words.end());
        string res;
        
        for(auto word:words){
            if(word.size() == 1 ||
               hash.count(word.substr(0,word.size()-1))){
                hash.insert(word);
                res = (word.size() > res.size())? word:res;
            }
        }
        return res;
        
    }
};

运行效率:

Runtime: 48 ms, faster than 83.47% of C++ online submissions for Longest Word in Dictionary.
Memory Usage: 18.7 MB, less than 76.12% of C++ online submissions for Longest Word in Dictionary.