LintCode 链表倒数第n个节点

/**
* 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
int sum=0;
ListNode *r;
r=head;
while(r!=NULL)
{
sum++;
r=r->next;
}
sum=sum-n+1;
r=head;
while(sum>1)
{
r=r->next;
sum--;
}
return r;
}
};