有老熊、小熊、老虎、小虎、老豹、小豹要过河，河边只有一条船，因船小，每船最多只能坐一个划手和一个乘客，动物中只有老熊、老虎、老豹和小熊能划船，当老兽不在时，它的小兽就要被其他老兽吃掉，请设计一个程序，让全部动物尽快过河。-There which stay, bear, tiger, tiger, leopard and old, small leopard to cross the river, the river is only one boat, due to small boat, each boat can only take one arm stroke and one passenger, which stay only animals, tigers, old leopard and the bear can be boating, when the old beast is not, it will be other small animals eat the old beast, you design a program that allows all animals to cross the river as soon as possible.