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：
1 struct ListNode 2 { 3 int val; 4 ListNode *next; 5 ListNode(int x) : val(x), next(NULL) {} 6 7 }; 8 ListNode *swapPairs(ListNode *head) 9 { 10 if (head == NULL || head->next == NULL) 11 return head; 12 ListNode *p = head; 13 ListNode *q = p->next; 14 while (q) 15 { 16 int temp; 17 temp = p->val; 18 p->val = q->val; 19 q->val = temp; 20 if (q->next != NULL) 21 { 22 p = q->next; 23 if (p->next != NULL) 24 q = p->next; 25 else 26 break; 27 28 } 29 else 30 break; 31 } 32 return head; 33 }