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;
}