题解 | #链表内指定区间反转#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     *
     * @param head ListNode类
     * @param m int整型
     * @param n int整型
     * @return ListNode类
     */
    public ListNode reverseBetween (ListNode head, int m, int n) {
        if (m == n) {
            return head;
        }
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode prev = dummy;

        // 开始需要翻转的头结点
        for (int i = 1; i < m; i++) {
            prev = prev.next;
        }
        // 要翻转的头结点
        ListNode midHead = prev.next;

        // 右侧不动的头结点
        ListNode tail = midHead;
        for (int i = m; i <= n; i++) {
            tail = tail.next;
        }

        // 翻转mid，这里把preTmp指向tail 主要是把他当做前节点，反转后，就在尾部了
        ListNode preTmp = tail;
        ListNode cur = midHead;
        while (cur != tail) {
            ListNode next = cur.next;
            cur.next = preTmp;
            preTmp = cur;
            cur = next;
        }
        prev.next = preTmp;
        return dummy.next;
    }
}