class Solution {
public:
    string findLongestWord(string s, vector<string>& dictionary) {
        if (dictionary.size() == 0) return "";
        string res = "";
        for (auto sub : dictionary)
        {
            if (issubstr(s, sub) == true)
                if (sub.size() > res.size() || sub.size() == res.size() && sub < res)
                    res = sub;
        }
        return res;
    }

    bool issubstr(string s, string dictionary)
    {
        int i, j;
        for (i = 0, j = 0; i < s.size(); i ++ )
        {
            if (s[i] == dictionary[j]) j ++ ;
            if (j == dictionary.size()) break;
        }
        if (j >= dictionary.size()) return true;
        return false;
    }
};

改进

class Solution {
public:
    string findLongestWord(string s, vector<string>& dictionary) {
        //if (dictionary.size() == 0) return "";
        string res = "";
        for (auto sub : dictionary)
        {
            if (issubstr(s, sub) == true)
                if (sub.size() > res.size() || sub.size() == res.size() && sub < res)
                    res = sub;
        }
        return res;
    }

    bool issubstr(string s, string dictionary)
    {
        int i, j;
        for (i = 0, j = 0; i < s.size(); i ++ )
            if (s[i] == dictionary[j]) j ++ ;
        return j == dictionary.size();
    }
};