LeeCode-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.

 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     else
32     {
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 }