[leetcode 24] Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

[Solution]

 1 ListNode *swapPairs(ListNode *head) 
 2     {
 3         ListNode dummy = ListNode(0), *p = &dummy, *q = head, *r = NULL;
 4         dummy.next = head;
 5         if (head == NULL || head->next == NULL)
 6             return head;
 7             
 8         r = q->next;
 9         while (r != NULL)
10         {
11             q->next = r->next;
12             r->next = q;
13             p->next = r;
14             p = q;
15             q = q->next;
16             r = (q == NULL) ? NULL : q->next;
17         }
18         
19         return dummy.next;
20     }