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) {
        // write code here
        ListNode pre = new ListNode(0);
        pre.next = head;
        //找到第一个节点的前一个节点
        ListNode f0 = pre;
        for(int i = 0; i < m - 1; i++){
            f0 = f0.next;
        }
        //找到第二个节点的当前节点
        ListNode f2 = pre;
        for(int i = 0; i < n ; i++){
            f2 = f2.next;
        }
        //第一个节点
        ListNode f1 = f0.next;
        //第二个节点的下一个节点
        ListNode f3 = f2.next;
        
        //断开链表
        f0.next = null;
        f2.next = null;
        
        //反转
        reverse(f1);
        
        //拼接
        f0.next = f2;
        f1.next = f3;
        return pre.next;
    }
    
    public ListNode reverse(ListNode head){
        ListNode pre = null;
        ListNode cur = head;
        while(cur != null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}