1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     void reverseList(ListNode *&head) {
12         ListNode *h = new ListNode(0);
13         ListNode *tmp;
14         while (head != NULL) {
15             tmp = head->next;
16             head->next = h->next;
17             h->next = head;
18             head = tmp;
19         }
20         head = h->next;
21     }
22
23     void twistList(ListNode *&l1, ListNode *&l2) {
24         ListNode *p1, *p2, *tmp;
25         p1 = l1; p2 = l2;
26         while (p1 != NULL && p2 != NULL) {
27             tmp = p2->next;
28             p2->next = p1->next;
29             p1->next = p2;
30             p1 = p1->next->next;
31             p2 = tmp;
32         }
33     }
34
35     void reorderList(ListNode *head) {
36         if (head == NULL || head->next == NULL || head->next->next == NULL) {
37             return;
38         }
39         ListNode *slow, *fast;
40         slow = head; fast = head;
41         while (fast != NULL && fast->next != NULL) {
42             slow = slow->next;
43             if (fast->next->next == NULL) {
44                 break;
45             }
46             fast = fast->next->next;
47         }
48         ListNode *l2 = slow->next;
49         slow->next = NULL;
50         reverseList(l2);
51         twistList(head, l2);
52     }
53 };