Leetcode|206/92. 反转链表 II（不断插入最左侧）

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* pre = nullptr;
        ListNode* cur = head;
        while (cur) {
            auto next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
};

class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int left, int right) {
        auto preHead = new ListNode();
        preHead->next = head;  // 容易遗忘
        auto pre = preHead;
        // 1.找到反转链表起始索引左边的节点
        for (int i = 0; i < left - 1; i++)
            pre = pre->next;
        ListNode* cur = pre->next;
        ListNode* next = nullptr;
        // 2.pre和cur不动，只把cur->next插入到pre->next后即可
        for (int i = 0; i < right - left; i++) {
            next = cur->next;
            cur->next = next->next;
            next->next = pre->next;
            pre->next = next;
        }
        return preHead->next;
    }
};