ｐｒｏｂｌｅｍ：

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.

ｔｈｉｎｋｉｎｇ：

（１）這道題在不新建結點的情況下，指向關係複雜，慢慢分析

（２）ｈｅａｄ是頭指標，指向第一個有效結點！！！

ｃｏｄｅ：

/**
 * 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;
    }
};