LintCode 166. 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
        ListNode* pre,*p;
        pre=head,p=head;
        while(n--)
        {
            if(p)
            {
                p=p->next;
            }
            else
            {
                return NULL;
            }
        }
        while(p)
        {
            p=p->next;
            pre=pre->next;
        }
        return pre;
    }
};