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

 #!/usr/bin/env python

 class Node(object):
     def __init__(self,elem,next_=None):
         self.elem = elem
         self.next = next_

 class Simple_List(object):
     def __init__(self):
         self.head = None

     def is_empty(self):
         return self.head == None

     def prepend(self,elem):
         self.head = Node(elem, self.head)

     def prepop(self):
         if self.is_empty():
             raise ValueError("list is empty")
         e = self.head.elem
         self.head = self.head.next
         return e

 def find_the_key(sl,key):
     if key < 0:
         raise ValueError("key must >= 0")
     i,j = sl.head,sl.head
     while key:
         j = j.next
         if j is None:
             raise ValueError("key must < list's length")
         key -= 1
     while j.next:
         i,j = i.next,j.next
     return i.elem

 if __name__=="__main__":
     sl = Simple_List()
     for i in range(5,-1,-1):
         sl.prepend(i)
     print(find_the_key(sl,0))