has a boat and three things: a dog, a chicken and a basket of chicken feed. He
wants to cross a river, but each time he crosses he can only bring one of his
things across. If he leaves the dog and chicken alone, the dog will eat the
chicken. If he leaves the chicken and the chicken feed alone, the chicken will
eat the feed. What is the least number of one-way trips across the river that
the man needs to cross with all of his things intact?