24. 两两交换链表中的节点

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9  //这题对我来说很难 
10 class Solution 
11 {
12 public:
13     ListNode* swapPairs(ListNode* head) 
14     {
15         ListNode* dummy = new ListNode(-1);
16         dummy -> next = head;
17         ListNode* cur = dummy;
18 
19         while (cur != NULL) 
20         {
21             ListNode* first = cur -> next;
22             if (first == NULL) break;
23 
24             ListNode* second = first -> next;
25             if (second == NULL) break;
26 
27             // 按照一定的次序，交换相邻的两个结点。
28             cur -> next = second;
29             first -> next = second -> next;
30             second -> next = first;
31 
32             cur = first;
33         }
34         return dummy -> next;
35     }
36 };