python 2.6 引入了itertools模块，使得排列组合的实现非常简单：
import itertools
排列：e.g., 4个数内选2个排列
>>> print list(itertools.permutations([1,2,3,4],2)) [(1, 2), (1, 3), (1, 4), (2, 1), (2, 3), (2, 4), (3, 1), (3, 2), (3, 4), (4, 1), (4, 2), (4, 3)]
组合：e.g.,4个数内选2个
>>> print list(itertools.combinations([1,2,3,4],2)) [(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]