The family intends to travel to Northern Europe together, and each expresses the following wishes: Father: If they go to Norway, they will not go to Denmark and Iceland; Mother: If they do not go to Iceland, they will go to Norway and Denmark; Son: If they do not go to Norway, they will go to Sweden and Iceland. Finland.
The final solution fulfills the wishes of each of the above, which can be concluded to be Sweden, Finland and Iceland.