92. Reverse Linked List II（链表部分反转）

Reverse a linked list from position m to n. Do it in-place and in one-pass.

For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,

return 1->4->3->2->5->NULL.

Note:
Given m, n satisfy the following condition:
1 ≤ m ≤ n ≤ length of list.

class Solution {
   public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null) return head;
        ListNode fakehead = new ListNode(0);
        fakehead.next = head;
        ListNode pre  = fakehead;
        
       
        for (int i = 0; i < m-1;i++) pre = pre.next;
        
        ListNode start = pre.next;
        ListNode then = start.next;
        
        for(int i = 0 ;i < n -m ;i++){
            start.next= then.next;
            then.next = pre.next;
            pre.next = then;
            then=start.next;
        }
    return fakehead.next;
    }
}