汉诺塔算法：
>>> def hnt(n,a,b,c):
... if n==1:
... print(a,'-->',c)
... else:
... hnt(n-1,a,c,b)
... hnt(1,a,b,c)
... hnt(n-1,b,a,c)
...
>>> hnt(3,'A','B','C')
A --> C
A --> B
C --> B
A --> C
B --> A
B --> C
A --> C
