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.

这道题太简单了，不值得一提·········

/**
 * 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 *p1,*p2,*tail;
       
        p1=head;
        p2=head->next;
        p1->next=p2->next;
        p2->next=p1;
        head=p2;           //得到头结点
        tail=p1;
        while(tail->next!=NULL&&tail->next->next!=NULL)
        {
            p1=tail->next;
            p2=p1->next;
            tail->next=p2;
            p1->next=p2->next;
            p2->next=p1;
            tail=p1;
        }
        return head;
        
        
    }
};