/**
 * 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) {
        if (left == right || !head->next) {
            return head;
        }

        auto dummy = new ListNode(-1);
        dummy->next = head;

        auto a = dummy;
        for (int i = 0; i< left -1; ++i) {
            a = a->next;
        }
        auto b = a->next;
        auto c = b->next;
        auto d = c->next;
        for (int i = 0; i< right- left; ++i) {
            d = c->next;
            c->next = b;
            b = c;
            c = d;
        }
        a->next->next = c;
        a->next = b;
        return dummy->next;
    }
};