92.反转链表-ii.cpp

/*
 * @lc app=leetcode.cn id=92 lang=cpp
 *
 * [92] 反转链表 II
 */

// @lc code=start
/**
 * 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* successor = nullptr;  // 后驱节点
    // 反转以 head 为起点的 n 个节点，返回新的头节点
    ListNode* reverseN(ListNode* head, int n) {
        if (n == 1) {
            // 记录第 n+1 个节点
            successor = head->next;
            return head;
        }
        // 以 head.next 为起点，反转前 n-1 个节点
        ListNode* last = reverseN(head->next, n-1);
        head->next->next = head;
        head->next = successor;
        return last;
    }

    ListNode* reverseBetween(ListNode* head, int left, int right) {
        // base case
        if (left == 1)
            return reverseN(head, right);
        // 前进到反转的起点触发 base case
        head->next = reverseBetween(head->next, left-1, right-1);
        return head;
    }
};
// @lc code=end