leetcode92 反转链表II

思路：

这里记录一种递归实现的方法，参考了https://zhuanlan.zhihu.com/p/86745433。

实现：

/**
 * 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* tail = NULL;
    ListNode* reverseN(ListNode* head, int N)
    {
        if (N == 1) { tail = head->next; return head; }
        ListNode* res = reverseN(head->next, N - 1);
        head->next->next = head;
        head->next = tail;
        return res; 
    }
    ListNode* reverseBetween(ListNode* head, int left, int right)
    {
        if (left == 1) return reverseN(head, right);
        head->next = reverseBetween(head->next, left - 1, right - 1);
        return head;
    }
};