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;
        
    }