一部之内可以多次调用统一规则!

for (int i = 1; i <= len; i++){
   int pos = s.find(a[i]);
   while (pos != string::npos){
      s = s.substr(0, pos) + b[i] + s.substr(pos + a[i].length(), s.size());
      q.push(s);
      st.push(step + 1);
      s = tmp;
      pos = s.find(a[i], pos + 1);
      }
}