Specified interval inversion in the linked list

Topic link ：https://www.nowcoder.com/practice/b58434e200a648c589ca2063f1faf58c

  analysis ：

The original list ：m = 2 ,n = 4 

After reversing  

1、 Need to get to m Start node

2、 Flip m - n The node of

3、m Node next Pointer to n Node next

4、m−1 Node next Pointer to n node

5、 If m=1, At this time ,m-1 Nodes don't exist , Add an empty node before the head node of the linked list to point to the head node

Code ：

import java.util.*;


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


public class Solution {
    /**
     * 
     * @param head ListNode class  
     * @param m int integer  
     * @param n int integer  
     * @return ListNode class 
     */
    public ListNode reverseBetween (ListNode head, int m, int n) {
        if(m==n){
            return head;
        }
        // write code here
        ListNode d = new ListNode(-1);
        d.next = head;
        ListNode pre = d;// Precursor node to flip 
        for(int i=1;i<m;i++){
            pre = pre.next;
        }
        head = pre.next;//m head 
        ListNode next; // For transit 
        //m-n Flip 
        for(int i=m; i<n;i++){
            next = head.next;
            head.next = next.next;
            next.next = pre.next;
            pre.next = next;
        }
        return d.next;
    }
}