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.

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *swapPairs(ListNode *head) {
if(!head||!head->next)
return head;
//申请哨兵节点
ListNode *h = (ListNode*)malloc(sizeof(struct ListNode));
h->next=head;
ListNode *s,*p,*q,*tmp;
s=h;
//两个节点中,p作为第一个节点,s是p前一个节点
p=s->next;
while(p){
//q作为第二个节点
q=p->next;
if(!q)
break;
p->next=q->next;
s->next=q;
q->next=p; s=p;
p=p->next;
}
p=h->next;
free(h);
return p;
}
};

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