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.

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     struct ListNode *next;
  * };
  */
 struct ListNode* swapPairs(struct ListNode* head)
 {
         struct ListNode *p=head,*q=head;

     int LiskLen=;

     while(q!=NULL)
     {
         LiskLen++;
         q=q->next;
     }
     if(LiskLen%==)
     {
         while(p!=NULL)
         {
             int temp;
             temp=p->val;
             p->val=p->next->val;
             p->next->val=temp;

             p=p->next->next;
         }
     }
     else
     {
         while(p->next!=NULL)
         {
             int temp;
             temp=p->val;
             p->val=p->next->val;
             p->next->val=temp;

             p=p->next->next;
         }
     }

     return head;
 }