P117: Split a list into two parts; the length of the first part is given
给定第一部份列表长度，将列表切分为两部份。照例还是先写测试用例：
from python99.lists.p117 import split
def test_split():
assert split([1,2,3,4,5,6],0) == ([],[1,2,3,4,5,6])
assert split([1,2,3,4,5,6],-1) == ([], [1,2,3,4,5,6])
assert split([1,2,3,4,5,6],2) == ([1,2],[3,4,5,6])
assert split([1,2,3,4,5,6],5) == ([1,2,3,4,5],[6])
assert split([1,2,3,4,5,6],6) == ([1,2,3,4,5,6],[])
assert split([1,2,3,4,5,6],7) == ([1,2,3,4,5,6],[])
Python内建的数据类型list已经提供了切片功能（Ninety-Nine Problems原来是为Prolog教学演示设计， Prolog中的list没有切片功能，也没有索引访问功能）。
代码实现：
# Split a list into two parts; the length of the first part is given.
def split(l, n):
if n <0:
return ([],l)
if n> len(l):
return (l,[])
return (l[0:n],l[n:len(l)])