题目：输入一个链表，输出该链表中倒数第 k 个结点。
思路：使用两个指针left，right都指向链表的头部head，然后right向后先走k-1步，接着left、right同时向后走直到right走到头，此时left就是链表倒数第k个节点
Python代码如下：
class listNode(object):
def __init__(self, val):
self.val = val
self.next = None
class Solution(object):
def FindKthToTail(self, head, k):
left = head
right = head
if (not head) or (k<=0):
return None
else:
for i in range(k-1):
if not right.next:
return None
else:
right = right.next
while right.next:
right = right.next
left = left.next
return left
s = Solution()
l1 = listNode('1')
l2 = listNode('2')
l3 = listNode('3')
l4 = listNode('4')
l1.next = l2
l2.next = l3
l3.next = l4
print s.FindKthToTail(l1, 2).val # 3