说明：  一个农夫在河边带了一只狼、一只羊和一颗白菜，他需要把这三样东西用船带到河的对岸。然而，这艘船只能容下农夫本人和另外一样东西。如果农夫不在场的话，狼会吃掉羊，羊也会吃掉白菜。请编程为农夫解决这个过河问题。
(A farmer brought a wolf, a sheep and a cabbage by the river. He needed to take the three things to the other side of the river by boat. However, the boat could only accommodate the farmer himself and something else. If the farmer is absent, the wolf will eat the sheep and the sheep will eat the cabbage. Please program for farmers to solve the problem of crossing the river.)