/**
 * 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) {
        if(head==NULL)
            return NULL;
        ListNode *pPre=NULL;//找出m-1点的位置
        ListNode *pList=head;
        for(int i=1;i<m;i++)
        {
            pPre=pList;
            pList=pList->next;
        }
        ListNode *pLeft=pList; //m点位置
        ListNode *pRight=pList;//找出n点的位置
        ListNode *pTail;//找出n+1点的位置
        for(int i=m;i<=n;i++)
        {
            pTail=pList->next;
            pList->next=pRight;
            pRight=pList;
            pList=pTail;
        }
        if(pPre==NULL)
        {
            head=pRight;
        }
        else
        {
            pPre->next=pRight;
        }
        pLeft->next=pTail;
        return head;
    }
};