1 struct ListNode {
 2     int val;
 3     ListNode *next;
 4     ListNode(int x) : val(x), next(NULL) {}
 5 };
 6 
 7 class Solution {
 8 public:
 9     ListNode *removeNthFromEnd(ListNode *head, int n) {
10         if (head == NULL) return NULL;
11 
12         ListNode *pre = head, *slow = head, *quick = head;
13         for (int i = 1; i <= n; i++) quick = quick->next;
14 
15         while (quick != NULL) {
16             pre = slow;
17             slow = slow->next;
18             quick = quick->next;
19         }
20         if (slow != head) {
21             pre->next = slow->next;
22             delete slow;
23             slow = NULL;
24         }
25         else {
26             head = slow->next;
27             delete slow;
28         }
29         return head;
30     }
31 };