public static SinglyLinkedListNode reverse(SinglyLinkedListNode llist) {
    
        SinglyLinkedListNode current = llist;
        SinglyLinkedListNode pre = null;

        while(current != null){
            SinglyLinkedListNode next = current.next;
            current.next = pre;
            pre = current;
            current = next;
        }
        return pre;
        
    }