翻转链表

 1 /**
 2  * Definition of ListNode
 3  * 
 4  * class ListNode {
 5  * public:
 6  *     int val;
 7  *     ListNode *next;
 8  * 
 9  *     ListNode(int val) {
10  *         this->val = val;
11  *         this->next = NULL;
12  *     }
13  * }
14  */
15 class Solution {
16 public:
17     /**
18      * @param head: The first node of linked list.
19      * @return: The new head of reversed linked list.
20      */
21     ListNode *reverse(ListNode *head) {
22         // write your code here
23         ListNode *l1=NULL,*l2=NULL,*l3=NULL;
24 
25         l1 = head; 
26         //  链表没有节点或有一个节点 
27         if(l1 == NULL || l1->next == NULL) {
28             return l1;
29         }
30         l2 = l1->next;
31         //  链表有2节点 
32         if(l2->next == NULL)  {
33             l2->next = l1;
34             l1->next = NULL;
35             return l2;
36         }
37         l3 = l2->next;
38         //  链表有3个以上节点
39         if(l2->next != NULL)  {
40             while(l2 != l3)   {
41                 l2->next = l1;
42                 if(l1 == head)
43                     l1->next = NULL;
44                 l1 = l2;
45                 l2 = l3;
46 
47                 if(l3->next != NULL)
48                     l3 = l3->next;
49             }
50             l2->next = l1;
51             return l2;
52         }
53     }
54 };
55