题目：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.
代码：
struct ListNode
{
int val;
ListNode *next;
ListNode(int x) : val(x), next(NULL) {} };
ListNode *swapPairs(ListNode *head)
{
if (head == NULL || head->next == NULL)
return head;
ListNode *p = head;
ListNode *q = p->next;
while (q)
{
int temp;
temp = p->val;
p->val = q->val;
q->val = temp;
if (q->next != NULL)
{
p = q->next;
if (p->next != NULL)
q = p->next;
else
break; }
else
break;
}
return head;
}