nth_to_last_node.cpp

/**
 * 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 *fast = head;
        ListNode *slow = head;
        for (int i = 0; i < n; i++) {
            if (head != NULL) {
                fast = fast->next;
            } else {
                return NULL;
            }
        }
        while(fast != NULL) {
            fast = fast->next;
            slow = slow->next;
        }
        return slow;
    }
};