题解 | #链表内指定区间反转#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @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) {
        // write code here
        if(head != null && head.next != null && m != n) {
            ListNode cur = head;
            int tm = m, tn = n - m;
            //找到需要翻转区间的起始端点（s，e），和端点区间左右相邻点（left，right）
            ListNode left = null;
            while(tm-->1) {
                left = cur;
                cur = cur.next;
            }
            ListNode s = cur;
            while(tn-->0) {
                cur = cur.next;
            }
            ListNode e = cur;
            ListNode right = cur.next;
            //链表翻转
            e.next = null;
            ListNode pre = null;
            cur = s;
            while(cur != null) {
                ListNode tmp = cur.next;
                cur.next = pre;
                pre = cur;
                cur = tmp;
            }
            //补齐: left->（e，s）->right
            if(left != null) {
                left.next = e;
            }
            s.next = right;
            //注意输出表头
            return left == null ? e : head;
        }
        return head;
    }
}