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.

#include<stdio.h>
#include<stdlib.h>

typedef struct ListNode {
    int val;
    struct ListNode *next;
 }*ListNode;

ListNode *swapPairs(ListNode *head) {
    ListNode *p1,*p2,*p3;
    int n=0;
    for(p1=p2=p3=head;p1!=NULL && p1->next!=NULL;n++){
        p1=p1->next;
        p2->next=p1->next;
        p1->next=p2;
        if(n!=0) p3->next=p1;
        else head=p1;
        p3=p2;
        p1=p2=p2->next;
    }
    return head;
}

Swap Nodes in Pairs