/**

  * 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) {

         int d=n-m;

         if(head==NULL||head->next==NULL||d<=)

             return head;

         ListNode* p=head,*bfp=p;

         ListNode* q=head;

         for(int i=;i<m-;i++){

             bfp=p;

             p=p->next;

         }

         ListNode* revhead=p;

         ListNode* bf=p,*bh=NULL;

         p=p->next;

         d--;

         while(p->next!=NULL&&d!=){

             d--;

             bh=p->next;

             p->next=bf;

             bf=p;

             p=bh;

         }

         bh=p->next;

         p->next=bf;

         bfp->next=p;

         revhead->next=bh;

         if(m!=)

             return head;

         else return p;

     }

 };