771. 宝石与石头/C++

int numJewelsInStones(string J, string S) {
    set<char> hashset;
    for(char c:J)
        hashset.insert(c);
    
    int num=0;
    for(char c:S)
        if(hashset.count(c)!=0)
            ++num;
      
    return num;
}