Reverse Linked List II

 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 class Solution {
10 public:
11     ListNode* reverseBetween(ListNode* head, int m, int n) {
12         if(m==n) return head;      //经常弄成赋值，找bug找半天
13         int i=1;
14         ListNode prehead(0);      // 注意进行初始化
15         prehead.next=head;
16         ListNode *mpre,*p;
17       
18         mpre=&prehead;
19       
20       while(i<m)
21       {
22           if(head&&head->next)
23           {
24              head=head->next;
25              mpre=mpre->next;
26              i++; 
27           }
28           else return prehead.next;
29           
30       }
31       
32         p=head->next;
33    
34         while(i<n)
35         {
36             if(p)
37             {
38                 p=head->next;
39                 head->next=p->next;
40                 p->next=mpre->next;
41                 mpre->next=p;
42                 i++;
43             }
44             else break;
45         }
46 
47         return prehead.next;
48         
49     }
50     
51 };