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

代码

class Solution {
public:
    ListNode *reverseBetween(ListNode *head, int m, int n) {
        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *cur = dummy;
        ListNode *pre, *front, *last;
        for (int i = 1; i <= m - 1; ++i) cur = cur->next;
        pre = cur;
        last = cur->next;
        for (int i = m; i <= n; ++i) {
            cur = pre->next;
            pre->next = cur->next;
            cur->next = front;
            front = cur;
        }
        cur = pre->next;
        pre->next = front;
        last->next = cur;
        return dummy->next;
    }
};