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.

思路：这道题主要就是依次进行两个结点的判断，关键因素就是这两个结点的前一个结点，使用变量pRev,pNode,pNext;主要就是对pRev的把握。

/**
 * 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 *pNode=head;
        ListNode *pNext=NULL;
        ListNode *pRev=head;
        while(pNode&&pNode->next)
        {
            pNext=pNode->next;
            pNode->next=pNext->next;
            pNext->next=pNode;
            if(pRev!=head)
                pRev->next=pNext;
            else
                head=pNext;
            pRev=pNode;
            pNode=pNode->next;
        }
        return head;
    }
};