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    1. /**
    2.  * Definition for singly-linked list.
    3.  * struct ListNode {
    4.  *     int val;
    5.  *     ListNode *next;
    6.  *     ListNode() : val(0), next(nullptr) {}
    7.  *     ListNode(int x) : val(x), next(nullptr) {}
    8.  *     ListNode(int x, ListNode *next) : val(x), next(next) {}
    9.  * };
    10.  */
    11. class Solution {
    12. public:
    13.     ListNode* reverseBetween(ListNode* head, int left, int right) {
    14.         ListNode *dummy =new ListNode(-1);
    15.         dummy->next = head;
    16.         ListNode *prev = dummy; 
    17.         for(int i = 0; i < left - 1; i ++) prev = prev->next;
    18.         ListNode *l = prev->next;
    19.         ListNode *r = l->next;
    20.         for(int i = 0; i < (right - left); i ++){
    21.             ListNode *next = r->next;
    22.             r->next = l;
    23.             l = r, r = next;
    24.         }
    25.         prev->next->next = r;
    26.         prev->next = l;
    27.         return dummy->next;
    28.     }
    29. };