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。
样例
给出链表 3->2->1->5->null和n = 2，返回倒数第二个节点的值1.
/**
* Definition of ListNode
* class ListNode {
* public:
* int val;
* ListNode *next;
* ListNode(int val) {
* this->val = val;
* this->next = NULL;
* }
* }
*/
class Solution {
public:
/**
* @param head: The first node of linked list.
* @param n: An integer.
* @return: Nth to last node of a singly linked list.
*/
ListNode *nthToLast(ListNode *head, int n) {
// write your code here
if(!head||!head->next) return head;
int res=;
ListNode *p=head;
while(p){
res++;
p=p->next;
}
ListNode *q=head;
for(int i=;i<=res;i++){
if(res-i+==n){
return q;
}
q=q->next;
}
}
};