```
/**
* 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 || head->next == NULL) return head;
ListNode* p = head,*pq = NULL,*pm = NULL;
int i = 1;
if(m==1)
{
pq = p->next;
while(++i<=n)
{
pm=pq->next;
pq->next = p;
p=pq;
pq=pm;
}
head->next=pq;
return p;
}
else
{//1->2->3->4->5->NULL here, pa ->1 p->2,
ListNode*pa = head;
int i = 1;
while(++i<m)
pa = pa->next;
p = pa->next;
ListNode*pc=p;
pq = p->next;
i = 0;
while(++i<=n-m)
{
pm=pq->next;
pq->next = p;
p=pq;
pq=pm;
}
pc->next = pq;
pa->next=p;
return head;
}
}
};`enter code here`
```