两两交换链表中的节点
    题目链接：https://leetcode-cn.com/problems/swap-nodes-in-pairs/
    图片.png

    思路：三指针，考虑节点总数为奇数或偶数的情况。

    参考代码：

    1. class Solution {
    2. public:
    3.     ListNode* swapPairs(ListNode* head) {
    4.         if (!head || !head->next) {
    5.             return head;
    6.         }
    7.         ListNode* pre = head;
    8.         ListNode* curr = head->next;
    9.         ListNode* temp = curr->next;
    10.         head = curr;
    11.         while (pre) {
    12.             curr->next = pre;
    13.             pre->next = temp;
    14.             if (!pre->next || !pre->next->next) {
    15.                 break;
    16.             }
    17.             pre->next = pre->next->next;
    18.             pre = temp;
    19.             curr = pre->next;
    20.             temp = curr->next;
    21.         }
    23.         return head;
    24.     }
    25. };