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。

示例:

输入: 1->2->3->4->5->NULL, m = 2, n = 4
输出: 1->4->3->2->5->NULL

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *reverseBetween(ListNode *head, int m, int n) {
        if (m == n) return head;

        auto dummy = new ListNode(-1);
        dummy->next = head;

        auto a = dummy, d = dummy;
        while (--m) a = a->next;
        while (n--) d = d->next;

        auto b = a->next, c = d->next;
        auto p = b, q = b->next;
        while (q != c) {
            auto o = q->next;
            q->next = p;
            p = q;
            q = o;
        }

        a->next = d, b->next = c;
        return dummy->next;
    }
};