leetcode 题解 ||　Swap Nodes in Pairs 问题

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) {
ListNode *index = head;
ListNode *pre = NULL;
ListNode *tmp = NULL;
ListNode *modify = head;;
int i=0;
if(head==NULL||head->next==NULL)
return head;
while((index!=NULL)&&(index->next!=NULL))
{
i++;
pre=index;
if(i==1)
{
head=pre->next;
index = index->next;
tmp = index->next;
pre->next = tmp;
index->next = pre;
index=tmp;
modify=pre;
}
else
{
index = index->next;
tmp = index->next;
modify->next=pre->next;
pre->next = tmp;
index->next = pre;
index=tmp;
modify=pre;

}

}
return head;
}
};