ReverseBetween.java

public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null) 
             return null;

        ListNode dummy = new ListNode(0); 
        dummy.next = head;
        ListNode pre = dummy; 
        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 dummy.next;
}