参考代码:

class ListNode(object):
    def __int__(self, val):
        self.val = val
        self.next = None

class Solution:
    def nthToLast(self, head, n):
        if head is None or n < 1:
            return None
        cur = head.next
        while cur is not None:
            cur.pre = head
            cur = cur.next
            head = head.next
        n -= 1
        while n > 0:
            head = head.pre
            n -= 1
        return head