leetcode 92. 反转链表 II

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

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-linked-list-ii
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if (m == n) return head;
        ListNode first = new ListNode(-1);
        first.next = head;

        ListNode last = first;
        ListNode p = first, q = null, t = null, b = first;
        for (int i = 0; i < m-1; i++) {
            b = b.next;
        }
        p = b.next;
        for (int i = 0; i <= n; i++) {
            last = last.next;
        }
        q = p.next;
        p.next = last;
        for (; q != last; p = q, q = t) {
            t = q.next;
            q.next = p;
        }
        b.next = p;
        return first.next;
    }
}