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    Simple java solution TC-O(N) SC-O(1)

    public static DoublyLinkedListNode reverse(DoublyLinkedListNode llist) {
            if(llist == null || llist.next == null){
                return llist;
            }
            DoublyLinkedListNode last = null;
            DoublyLinkedListNode curr = llist;
            
            while(curr!=null){
                last = curr.prev;
                curr.prev = curr.next;
                curr.next = last;
                curr = curr.prev;
            }
            return last.prev;
        }
    
    }