编程题：求单链表倒数第k位的值（最后一位为倒数第0位）好未来

 1 #!/usr/bin/env python
 2 
 3 class Node(object):
 4     def __init__(self,elem,next_=None):
 5         self.elem = elem
 6         self.next = next_
 7 
 8 class Simple_List(object):
 9     def __init__(self):
10         self.head = None
11 
12     def is_empty(self):
13         return self.head == None
14 
15     def prepend(self,elem):
16         self.head = Node(elem, self.head)
17 
18     def prepop(self):
19         if self.is_empty():
20             raise ValueError("list is empty")
21         e = self.head.elem
22         self.head = self.head.next
23         return e
24 
25 def find_the_key(sl,key):
26     if key < 0:
27         raise ValueError("key must >= 0")
28     i,j = sl.head,sl.head
29     while key:
30         j = j.next
31         if j is None:
32             raise ValueError("key must < list's length")
33         key -= 1
34     while j.next:
35         i,j = i.next,j.next
36     return i.elem
37 
38 if __name__=="__main__":
39     sl = Simple_List()
40     for i in range(5,-1,-1):
41         sl.prepend(i)
42     print(find_the_key(sl,0))