public static class MyLinkedList {
        protected Element head;
        protected Element tail;

        void insertAt(int n){

        }
        public Element find(int mydata){
            Element starter = head;
            while(starter != null){
                if(starter.data == mydata){
                    return starter;
                }
                starter = starter.next;
            }
            return null;
        }
        public String toString(){
            String result = "[ ";
            Element starter = head;
            while(starter != tail){
                result = result + String.valueOf(starter.data) + ", ";
                starter = starter.next;
            }
            result = result + String.valueOf(starter.data);
            result += " ]";
            return result;
        }
        public void append(int mydata) {
            Element element = new Element(mydata, null);
            if (head == null) head = element;
            else tail.next = element;
            tail = element;
        }

        public void prepend(int mydata) {
            Element element = new Element(mydata, head);
            if (head == null) tail = element;
            head = element;
        }

        public int countNodes() {
            int count = 0;
            Element e = head;
            while (e != null) {
                count++;
                e = e.next;
            }
            return count;
        }

        public int countNodes(int mydata) {
            int count = 0;
            Element e = head;
            while (e != null) {
                if (e.data == mydata)
                    count++;
                e = e.next;
            }
            return count;
        }

        public void extractFirst() {
            if (head == null) throw new IllegalArgumentException("item not found");
            head = head.next;
            if (head == null) tail = null;
        }

        public void extractLast() {
            if (tail == null) throw new IllegalArgumentException("item not found");
            if (head == tail) head = tail = null;
            else {
                Element previous = head;
                while (previous.next != tail) previous = previous.next;
                previous.next = null;
                tail = previous;
            }
        }

        public void extract(int mydata) {
            Element element = head;
            Element previous = null;
            while (element != null && !(element.getData() == mydata)) {
                previous = element;
                element = element.next;
            }
            if (element == null) throw new IllegalArgumentException("item not found");
            if (element == head) head = element.next;
            else previous.next = element.next;
            if (element == tail) tail = previous;
        }

        public final class Element {
            int data;
            Element next;

            Element(int mydata, Element element) {
                data = mydata;
                next = element;
            }

            public int getData() {
                return data;
            }

            public Element getNext() {
                return next;
            }

            public void insertBefore(int mydata) {
                Element element = new Element(mydata, this);
                if (this == head) {
                    head = element;
                    return;
                }
                Element previous = head;
                while (previous.next != this) {
                    previous = previous.next;
                }
                previous.next = element;
            }

            public void insertAfter(int mydata) {
                next = new Element(mydata, next);
                if (this == tail)
                    tail = next;
            }
        }
    }