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=0;

    while(q!=NULL)
    {
        LiskLen++;
        q=q->next;
    }
    if(LiskLen%2==0)
    {
        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;
}