92.txt

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) 
    {
        if (head == nullptr || head->next == nullptr || m > n|| m < 0 || m > n || n < 0)  return head;
        
        ListNode prehead(0); //头节点之前的
        prehead.next = head;
        ListNode *cur = &prehead;
        for(int i = 1; i + 1 <= m; i++)
            cur = cur ->next ;  //cur =m-1

        ListNode *prefirst = cur; //m-1
        ListNode *first = cur->next; //m
        ListNode *pre = cur; //m-1
        ListNode *next;
        cur = cur ->next;

        for(int i = m; i <= n; i++)   //m-n 之前的掉顺序
        {
            next = cur->next;
            cur->next = pre;

            pre = cur;
            cur =next;



        }

        prefirst->next = pre;
        first->next  = cur;
    

        return prehead.next;
    }
};