RuntimeError: maximum recursion depth exceeded in cmp

python默认的递归深度是很有限的，如果碰到类似的错误，先行设置一下吧。

sys.setrecursionlimit(limit)

Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python.

The highest possible limit is platform-dependent. A user may need to set the limit higher when she has a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash.