python 排列组合之itertools

python 2.6 引入了itertools模块，使得排列组合的实现非常简单：

import itertools 

有序排列：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)]

无序组合：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)]