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
        if(m - n == 0) return head;
        int i = 1;
        ListNode l1 = new ListNode(0),l2 = new ListNode(0);
        ListNode p = head;
        while(head != null){
            //记录开始反转的前一个节点
            if(i == m - 1) l1 = head;
            if(i == n){
                //记录反转结尾的下一个节点
                l2 = head.next;
                break;
            }
            head = head.next;
            i++;
        }
        //开头反转，只需要连接反转结尾的节点
        if(m == 1){
            ListNode n1 = dfs(p,m,n);
            ListNode k = n1;
            while(n1.next != null){
                n1 = n1.next;
            }
            //连接反转结尾的节点
            n1.next= l2;
            return k;
        }else{
            ListNode k = dfs(l1.next,m,n);
            //连接开始反转的节点
            l1.next = k;
            while(k.next != null){
                k = k.next;
            }
            //连接结尾反转的节点
            k.next = l2;
        }

        return p;
    }
    //反转
    public ListNode dfs(ListNode p ,int m,int n){
        if(m == n){
            return p;
        }
        ListNode p1 = dfs(p.next,m + 1,n);
        p.next.next = p;
        p.next = null;
        return p1;
    }
}