class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        if (!head || !head->next) return head;
        ListNode *dummy = new ListNode(0), *p = dummy;
        dummy->next = head;

        for (int i = 1; i <= m - 1; ++i) 
            p = p->next;

        ListNode *mid = p;
        for (int i = 1; i <= n - m + 1; ++i) mid = mid->next;

        ListNode *post = mid->next; 
        mid->next = nullptr;

        mid = p->next;
        p->next = reverseList(p->next);
        mid->next = post;

        p = dummy->next;
        delete dummy; dummy = nullptr;
        return p;
    }

    // 翻转链表
    ListNode *reverseList(ListNode *head){
        if (!head || !head->next) return head;
        ListNode *pre = nullptr, *cur = head, *post = nullptr;
        while (cur){
            post = cur->next;
            cur->next = pre;
            pre = cur;
            cur = post;
        }
        return pre;
    }
};