For JAVA.

DoublyLinkedList.java: (Create this class and copy code into it)

public class DoublyLinkedList {

//---------------- nested Node class ---------------- /** * Node of a doubly linked list, which stores a reference to its * element and to both the previous and next node in the list. */ private static class Node {

/** The element stored at this node */ private E element; // reference to the element stored at this node

/** A reference to the preceding node in the list */ private Node prev; // reference to the previous node in the list

/** A reference to the subsequent node in the list */ private Node next; // reference to the subsequent node in the list

/** * Creates a node with the given element and next node. * * @param e the element to be stored * @param p reference to a node that should precede the new node * @param n reference to a node that should follow the new node */ public Node(E e, Node p, Node n) { element = e; prev = p; next = n; }

// public accessor methods /** * Returns the element stored at the node. * @return the element stored at the node */ public E getElement() { return element; }

/** * Returns the node that precedes this one (or null if no such node). * @return the preceding node */ public Node getPrev() { return prev; }

/** * Returns the node that follows this one (or null if no such node). * @return the following node */ public Node getNext() { return next; }

// Update methods /** * Sets the node's previous reference to point to Node n. * @param p the node that should precede this one */ public void setPrev(Node p) { prev = p; }

/** * Sets the node's next reference to point to Node n. * @param n the node that should follow this one */ public void setNext(Node n) { next = n; } } //----------- end of nested Node class -----------

// instance variables of the DoublyLinkedList /** Sentinel node at the beginning of the list */ private Node header; // header sentinel

/** Sentinel node at the end of the list */ private Node trailer; // trailer sentinel

/** Number of elements in the list (not including sentinels) */ private int size = 0; // number of elements in the list

/** Constructs a new empty list. */ public DoublyLinkedList() { header = new Node(null, null, null); // create header trailer = new Node(null, header, null); // trailer is preceded by header header.setNext(trailer); // header is followed by trailer }

// public accessor methods /** * Returns the number of elements in the linked list. * @return number of elements in the linked list */ public int size() { return size; }

/** * Tests whether the linked list is empty. * @return true if the linked list is empty, false otherwise */ public boolean isEmpty() { return size == 0; }

/** * Returns (but does not remove) the first element of the list. * @return element at the front of the list (or null if empty) */ public E first() { if (isEmpty()) return null; return header.getNext().getElement(); // first element is beyond header }

/** * Returns (but does not remove) the last element of the list. * @return element at the end of the list (or null if empty) */ public E last() { if (isEmpty()) return null; return trailer.getPrev().getElement(); // last element is before trailer }

// public update methods /** * Adds an element to the front of the list. * @param e the new element to add */ public void addFirst(E e) { addBetween(e, header, header.getNext()); // place just after the header }

/** * Adds an element to the end of the list. * @param e the new element to add */ public void addLast(E e) { addBetween(e, trailer.getPrev(), trailer); // place just before the trailer }

/** * Removes and returns the first element of the list. * @return the removed element (or null if empty) */ public E removeFirst() { if (isEmpty()) return null; // nothing to remove return remove(header.getNext()); // first element is beyond header }

/** * Removes and returns the last element of the list. * @return the removed element (or null if empty) */ public E removeLast() { if (isEmpty()) return null; // nothing to remove return remove(trailer.getPrev()); // last element is before trailer }

// private update methods /** * Adds an element to the linked list in between the given nodes. * The given predecessor and successor should be neighboring each * other prior to the call. * * @param predecessor node just before the location where the new element is inserted * @param successor node just after the location where the new element is inserted */ private void addBetween(E e, Node predecessor, Node successor) { // create and link a new node Node newest = new Node(e, predecessor, successor); predecessor.setNext(newest); successor.setPrev(newest); size++; }

/** * Removes the given node from the list and returns its element. * @param node the node to be removed (must not be a sentinel) */ private E remove(Node node) { Node predecessor = node.getPrev(); Node successor = node.getNext(); predecessor.setNext(successor); successor.setPrev(predecessor); size--; return node.getElement(); }

/** * Produces a string representation of the contents of the list. * This exists for debugging purposes only. */ public String toString() { StringBuilder sb = new StringBuilder("("); Node walk = header.getNext(); while (walk != trailer) { sb.append(walk.getElement()); walk = walk.getNext(); if (walk != trailer) sb.append(", "); } sb.append(")"); return sb.toString(); } } //----------- end of DoublyLinkedList class -----------

- Modify the doubly-linked list class provided at the very end of these instructions. Add a method findMiddle() that finds the middle node of a doubly linked list by link hopping, without relying on explicit knowledge of the size of the list. Counting nodes is not the correct way to do this. In the case of an even number of nodes, report the node slightly left of center as the middle. You can assume the list has at least one item. Your findMiddle() code will be located in DoublyLinkedList. java. If you're using size(), you're doing it wrong. Starter code for the findMiddle() method: public E findMiddle() \{ Node E middleNode = header. next; Node E currentNode = header.next; // the rest of your code goes here return middleNode.element; \} - You will test findMiddle() with the following three tests cases in a main class. Sample program output: Finding the middle of the list: (1,2,3,4,5,6,7) The middle value is: 4 Finding the middle of this list: (1,2,3,4) The middle value is: 2 Finding the middle of this list: (First, Second, Third, Fourth, Fifth) The middle value is: Third Your code must work for any doubly-linked list, not just the examples