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.

1 /** 2  * Definition for singly-linked list. 3  * struct ListNode { 4  *     int val; 5  *     struct ListNode *next; 6  * }; 7  */ 8 struct ListNode* swapPairs(struct ListNode* head)  9 {10         struct ListNode *p=head,*q=head;11 12     int LiskLen=0;13 14     while(q!=NULL)15     {16         LiskLen++;17         q=q->next;18     }19     if(LiskLen%2==0)20     {21         while(p!=NULL)22         {23             int temp;24             temp=p->val;25             p->val=p->next->val;26             p->next->val=temp;27             28             p=p->next->next;29         }30     }31     else32     {33         while(p->next!=NULL)34         {35             int temp;36             temp=p->val;37             p->val=p->next->val;38             p->next->val=temp;39             40             p=p->next->next;41         }42     }43 44     return head;45 }