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; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *swapPairs(ListNode *head) {    if(head==NULL||head->next==NULL)return head;    ListNode *res=new ListNode(-1);    res->next=head;    ListNode *front=head->next;    ListNode *later=head;    ListNode *p=res;    while(front!=NULL)    {        later->next=front->next;        front->next=later;        p->next=front;        p=later;        if (later->next==NULL)break;        later=later->next;        front=later->next;    }    return res->next;    }};