Force buckle 92 Reverse linked list II

/**
 * 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) return head;
        auto ans = new ListNode(-1, head), ptr = ans;
        for(int i = 0; i < left - 1; ++i)
            ptr = ptr->next;
        auto p1 = ptr->next;
        for(int i = 0; i < right - left; ++i)
        {
            auto temp = p1->next;
            p1->next= temp->next;
            temp->next = ptr->next;
            ptr->next = temp;
        }
        return ans->next;
    }
};