/**
 * 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* preHead=new ListNode(0);//添加一个头节点，后面省事多了
        preHead->next=head;
        ListNode* p=preHead,*tail=preHead;
        while(--m>0){
            p=p->next;
        }
        do{
            tail=tail->next;
        }while(n--);
        ListNode* q=p,*next=p->next;
        q->next=tail;
        p=next;
        while(p!=tail){  //头插法
            next=p->next;
            p->next=q->next;
            q->next=p;
            p=next;
        }
        return preHead->next;
    }
};