Leecode-swap Nodes in Pairs

Given a linked list, swap every, 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. 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 structlistnode* Swappairs (structlistnode*head)9 {Ten         structListNode *p=head,*q=head; One  A     intlisklen=0; -  -      while(q!=NULL) the     { -lisklen++; -Q=q->Next; -     } +     if(lisklen%2==0) -     { +          while(p!=NULL) A         { at             inttemp; -Temp=p->Val; -P->val=p->next->Val; -P->next->val=temp; -              -P=p->next->Next; in         } -     } to     Else +     { -          while(p->next!=NULL) the         { *             inttemp; $Temp=p->Val;Panax NotoginsengP->val=p->next->Val; -P->next->val=temp; the              +P=p->next->Next; A         } the     } +  -     returnhead; $}

Leecode-swap Nodes in Pairs