16_leetcode_swap node 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.

1:链表为NULL或者链表只有一个节点；2:首先交换头两个节点，确定头节点和尾节点；3:循环交换后两个节点；4当末尾节点个数不大于1的时候，直接添加到当前链表的尾部。

    ListNode* swapPairs(ListNode* head)    {        if(head == NULL || head->next == NULL)            return head;                ListNode* temp = head->next;        head->next = temp->next;        temp->next = head;        head = temp;        ListNode* index = head->next;        temp = index->next;                while(temp && temp->next)        {            index->next = temp->next;            index = index->next;            temp->next = index->next;            index->next = temp;            index = index->next;            temp = index->next;                    }        index->next = temp;                return head;            }