92. Reverse Linked List II

Reverse a linked list from position m to n. Do it in one-pass.

Note: 1 ≤ m ≤ n ≤ length of list.

Example:

Input: 1->2->3->4->5->NULL, m = 2, n = 4
Output: 1->4->3->2->5->NULL

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-linked-list-ii
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从指定的开始和结束进行倒置链表。

设置一个排头的空节点，next指向head，翻转只用按顺序：

① 1->2->3->4->5->NULL

② 1->3->2->4->5->NULL

③ 1->4->3->2->5->NULL

每次都把后一个放在前。

class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if (head.next == null) {
            return head;
        }
        ListNode beforeHead = new ListNode(-1);
        beforeHead.next = head;
        ListNode ahead = beforeHead;
        for (int i = 1; i < m; i++) {
            ahead = ahead.next;
        }
        ListNode nextHead = ahead.next;
        ListNode nextNextHead = nextHead.next;
        for (int i = 0; i < n - m; i++) {
            nextHead.next = nextNextHead.next;
            nextNextHead.next = ahead.next;
            ahead.next = nextNextHead;
            nextNextHead = nextHead.next;
        }
        return beforeHead.next;
    }
}