/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
const static int x=[]{
    std::ios::sync_with_stdio(false);
    std::cin.tie(NULL);
    return NULL;
}();
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if(m==n) return head;
        ListNode* cur_next;//當前遍歷節點的下一個節點
        ListNode* cur;     //當前遍歷節點
        ListNode* cur_prev; //當前遍歷節點的上一個節點
        ListNode* tail;    //反轉節點的最後一個節點
        ListNode* prev; 
        if(m==1){
            tail=head;
            cur=head;
            cur_prev=NULL;
            cur_next=cur->next;
            int i=m;
            for(int i=m;i<n+1;i++){
                cur->next=cur_prev;
                cur_prev=cur;
                cur=cur_next;
                if(cur!=NULL)
                    cur_next=cur->next;
            }
            head=cur_prev;
            tail->next=cur;
        }else{
            cur=head;
            int i=1;
            while(i<m-1){
                cur=cur->next;
                i++;
            }
            prev=cur;
            cur=cur->next;
            tail=cur;
            cur_prev=NULL;
            cur_next=cur->next;
            for(int i=m;i<n+1;i++){
                cur->next=cur_prev;
                cur_prev=cur;
                cur=cur_next;
                if(cur!=NULL)
                    cur_next=cur->next;
            }
            tail->next=cur;
            prev->next=cur_prev;
        }

        return head;
        
    }
};