反转链表II（中等）

反转从位置 m 到 n 的链表。请使用一趟扫描完成反转。

说明:

1 ≤ m ≤ n ≤ 链表长度。

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x): val(x), next(nullptr) {}
};
class Solution
{
public:
    ListNode* reverseBetween(ListNode *head, int m, int n)
    {
        if(n == 1 || !head) return head;
        ListNode *prev = nullptr;
        ListNode *curr = head;
        while(m > 1)
        {
            prev = curr;
            curr = curr->next;
            m--;
            n--;
        }
        ListNode *before = prev;
        ListNode *after = curr;
        while(n > 0)
        {
            ListNode *nextptr = curr->next;
            curr->next = prev;
            prev = curr;
            curr = nextptr;
            n--;
        }
        if(before) before->next = prev;
        else head = prev;
        after->next = curr;
        return head;
    }
};