leetcode. Reorder List

 1 void reorderList(ListNode *head) 
 2     {
 3         if (head == NULL || head->next == NULL || head->next->next == NULL)
 4             return;
 5         
 6         ListNode *p, *q, *dummy = new ListNode(INT_MIN);
 7         q = head;
 8         p = q->next;
 9         while (p != NULL)
10         {
11             q = q->next;
12             p = p->next;
13             if (p != NULL)
14                 p = p->next;
15         }
16         
17         dummy->next = q->next; // right half nodes
18         q->next = NULL;
19         q = head;   // left half nodes
20         reverse(dummy);
21         
22         while (q != NULL)
23         {
24             p = dummy->next;
25             dummy->next = p->next;
26             p->next = q->next;
27             q->next = p;
28             q = q->next;
29             q = q->next;
30             if (dummy->next == NULL)
31                 break;
32         }
33         delete dummy;
34     }
35     
36     void reverse(ListNode *dummy)
37     {
38         ListNode *p = dummy, *q, *r;
39         
40         q = p->next;
41         if (q == NULL)
42             return;
43         r = q->next;
44         while (r != NULL)
45         {
46             q->next = p;
47             p = q;
48             q = r;
49             r = r->next;
50         }
51         q->next = p;
52         dummy->next->next = NULL;
53         dummy->next = q;
54     }