倒数第k个节点
链表中倒数第k个结点
http://www.nowcoder.com/questionTerminal/529d3ae5a407492994ad2a246518148a
快慢指针
快指针先往前走k步，注意判断边界，然后快慢一起走，当快指针为none的时候，慢指针走到了倒数第k个节点
class Solution:
def FindKthToTail(self, head, k):
# write code here
slow,fast=head,head
for i in range(k):
if not fast:
return None
fast=fast.next
while fast:
slow=slow.next
fast=fast.next
return slow 
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