Leetcode -- Reverse Linked List II

 /**  
  * Definition for singly-linked list.  
  * struct ListNode {  
  *   int val;  
  *   ListNode *next;  
  *   ListNode(int x) : val(x), next(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   ListNode *reverseBetween(ListNode *head, int m, int n) {  
     ListNode * dummy = new ListNode(-1);  
     dummy->next = head;  
     ListNode * left = dummy;  
     ListNode * right = dummy;  
     for(int i = 0; i < m - 1; ++i) {  
       left = left->next;  
     }  
     ListNode * prevTail = left;  
     left = left->next;  
     for(int i = 0; i < n; ++i) {  
       right = right->next;  
     }  
     ListNode * nextHead = right->next;  
     right->next = NULL;  
     ListNode * prev = nextHead;  
     ListNode * curr = left;  
     ListNode * next = NULL;  
     while(curr != NULL) {  
       ListNode * next = curr->next;  
       curr->next = prev;  
       prev = curr;  
       curr = next;  
     }  
     prevTail->next = prev;  
     return dummy->next;  
   }  
 };