https://massivealgorithms.blogspot.com/2015/01/wolf-sheep-cabbage-algoritmynet.html
农夫每次可以带1-2个动物过河，河两岸每边如果狼的数量大于羊，则狼会吃掉羊
- https://www.youtube.com/watch?v=Sa9vbOdKUNg
- https://stackoverflow.com/questions/36247795/using-search-algorithm-in-python-to-solve-the-3-wolves-and-3-lambs-puzzle
- https://gamefaqs.gamespot.com/ds/936050-professor-layton-and-the-curious-village/answers/36362-how-do-i-solve-the-3-wolves-amp-3-sheep-puzzle