class Solution {
public:
ListNode reverseBetween(ListNode head, int m, int n) {
if(!head || !head->next || m==n)
return head;
ListNode dummy = new ListNode(0);
dummy->next = head;
ListNode cur = dummy;
for(int i = 0; i < m-1; ++i){
cur = cur->next;
}
ListNode* tail1 = cur;

    cur = cur->next;
    ListNode* tail2 = cur;

    ListNode* tmp1 = cur->next;
    ListNode* tmp2 = tmp1->next;
    for(int i = 0; i < n-m; ++i){
        tmp1->next = cur;

        cur = tmp1;
        tmp1 = tmp2;
        if(tmp2 == NULL)
            break;
        tmp2 = tmp2->next;
    }
    tail1->next = cur;
    tail2->next = tmp1;

    return dummy->next;
}

};