92

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int left, int right) {
        ListNode *start, *pre_ptr, *next_ptr;
        ListNode *dummy = new ListNode(0, head);
        ListNode *curr=dummy;
        for(int i=0; i<left-1; i++){
            curr = curr->next;
        }
        pre_ptr = curr;
        start = curr->next;
        next_ptr = start->next;

        for(int i=0; i<right-left; i++){
            start->next = next_ptr->next;
            next_ptr->next = pre_ptr->next;
            pre_ptr->next = next_ptr;
            next_ptr = start->next;
        }
        return dummy->next;
    }
};