166.h

/**
 * 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 == nullptr) return head;
        
        // 1
        // ListNode *dummyhead = head;
        // int length = 0;
        // while(dummyhead != nullptr){
        //     length++;
        //     dummyhead = dummyhead->next;
        // }
        
        // for(int i = 0; i < length - n; i++){
        //     head = head->next;
        // }
        // return head;
        
        // 2 两根指针
        int k = 0;
        ListNode *fast = head, *slow = head;
        while(k++ < n){
            fast = fast->next;
        }
        while(fast != nullptr){
            fast = fast->next;
            slow = slow->next;
        }
        return slow;
    }
};