链表倒数第n个节点

链表倒数第n个节点
找到单链表倒数第n个节点，保证链表中节点的最少数量为n。
题目地址
注意点：
1、dummy_node
2、n超出范围

 def nthToLast(self, head, n):
        # write your code here
        if n <= 0:
            return None
        dummuy_node = ListNode(0)
        dummuy_node.next = head
        fast = dummuy_node
        while n > 0 and fast:
            fast = fast.next
            n -= 1
        if n == 0:
            slow = dummuy_node
            
            while fast:
                fast = fast.next
                slow = slow.next
            return slow
        else:
            return None