[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]

ListNode *swapPairs(ListNode *head)
{
ListNode dummy = ListNode(0), *p = &dummy, *q = head, *r = NULL;
dummy.next = head;
if (head == NULL || head->next == NULL)
return head;

r = q->next;
while (r != NULL)
{
q->next = r->next;
r->next = q;
p->next = r;
p = q;
q = q->next;
r = (q == NULL) ? NULL : q->next;
}

return dummy.next;
}