Java双链表

/**
 * This class implements a DoublyLinkedList. This is done using the classes
 * LinkedList and Link.
 * 
 * A linked list is simplar to an array, it holds values. However,
 * links in a linked list do not have indees. With a linked list
 * you do not need to predetermine it's size as it grows and shrinks
 * as it is edited. This is an example of a double ended, doubly
 * linked list. Each link references the next link and the previous
 * one.
 * 
 * @author Unknown
 *
 */

class DoublyLinkedList{
    /** Head refers to the front of the list */
    private Link head;
    /** Tail refers to the back of the list */
    private Link tail;

    /**
     * Default Constructor
     */
    public DoublyLinkedList(){
        head = null;
        tail = null;
    }

        /**
    * Constructs a list containing the elements of the array
     * @param  array the array whose elements are to be placed into this list
    * @throws NullPointerException if the specified collection is null
    */
    public DoublyLinkedList(int[] array){
        if (array == null) throw new NullPointerException();
            for (int i:array) {
                insertTail(i);
        }
    }

    /**
     * Insert an element at the head
     * 
     * @param x Element to be inserted
     */
    public void insertHead(int x){
        Link newLink = new Link(x); //Create a new link with a value attached to it
        if(isEmpty()) //Set the first element added to be the tail
            tail = newLink;
        else
            head.previous = newLink; // newLink <-- currenthead(head)
        newLink.next = head; // newLink <--> currenthead(head)
        head = newLink; // newLink(head) <--> oldhead
    }

    /**
     * Insert an element at the tail
     * 
     * @param x Element to be inserted
     */
    public void insertTail(int x){
        Link newLink = new Link(x);
        newLink.next = null; // currentTail(tail)     newlink -->
        tail.next = newLink; // currentTail(tail) --> newLink -->
        newLink.previous = tail; // currentTail(tail) <--> newLink -->
        tail = newLink; // oldTail <--> newLink(tail) -->
    }

    /**
     * Delete the element at the head
     * 
     * @return The new head
     */
    public void deleteHead(){
        Link temp = head;
        head = head.next; // oldHead <--> 2ndElement(head)
        head.previous = null; // oldHead --> 2ndElement(head) nothing pointing at old head so will be removed
        if(head == null)
            tail = null;
    }

    /**
     * Delete the element at the tail
     * 
     * @return The new tail
     */
    public void deleteTail(){
        Link temp = tail;
        tail = tail.previous; // 2ndLast(tail) <--> oldTail --> null
        tail.next = null; // 2ndLast(tail) --> null

    }

    /**
     * Delete the element from somewhere in the list
     * 
     * @param x element to be deleted
     * @return  Link deleted
     */
    public void delete(int x){
        Link current = head;

        while(current.value != x) //Find the position to delete
            current = current.next;

        if(current == head)
            deleteHead();

        else if(current == tail)
            deleteTail();

        else{ //Before: 1 <--> 2(current) <--> 3
            current.previous.next = current.next;  // 1 --> 3
            current.next.previous = current.previous; // 1 <--> 3
        }   
    }

    /**
     * Inserts element and reorders
     * 
     * @param x Element to be added
     */
    public void insertOrdered(int x){
        Link newLink = new Link(x);
        Link current = head;
        while(current != null && x > current.value) //Find the position to insert
            current = current.next;

        if(current == head)
            insertHead(x);

        else if(current == null)
            insertTail(x);

        else{ //Before: 1 <--> 2(current) <--> 3
            newLink.previous = current.previous; // 1 <-- newLink
            current.previous.next = newLink; // 1 <--> newLink
            newLink.next = current; // 1 <--> newLink --> 2(current) <--> 3
            current.previous = newLink; // 1 <--> newLink <--> 2(current) <--> 3
        }
    }

    /**
     * Returns true if list is empty
     * 
     * @return true if list is empty
     */
    public boolean isEmpty(){
        return(head == null);
    }

    /**
     * Prints contents of the list
     */
    public void display(){ //Prints contents of the list
        Link current = head;
        while(current!=null){
            current.displayLink();
            current = current.next;
        }
        System.out.println();
    }
}

/**
 * This class is used to implement the nodes of the
 * linked list.
 * 
 * @author Unknown
 *
 */
class Link{
    /** Value of node */
    public int value;
    /** This points to the link in front of the new link */
    public Link next;
    /** This points to the link behind the new link */
    public Link previous;

    /**
     * Constructor
     * 
     * @param value Value of node
     */
    public Link(int value){
        this.value = value;
    }

    /**
     * Displays the node
     */
    public void displayLink(){
        System.out.print(value+" ");
    }

    /**
     * Main Method
     * 
     * @param args Command line arguments
     */
    public static void main(String args[]){
        DoublyLinkedList myList = new DoublyLinkedList();

        myList.insertHead(13);
        myList.insertHead(7);
        myList.insertHead(10);
        myList.display(); // <-- 10(head) <--> 7 <--> 13(tail) -->

        myList.insertTail(11);
        myList.display(); // <-- 10(head) <--> 7 <--> 13 <--> 11(tail) -->

        myList.deleteTail();
        myList.display(); // <-- 10(head) <--> 7 <--> 13(tail) -->

        myList.delete(7);
        myList.display(); // <-- 10(head) <--> 13(tail) -->

        myList.insertOrdered(23);
        myList.insertOrdered(67);
        myList.insertOrdered(3);
        myList.display(); // <-- 3(head) <--> 10 <--> 13 <--> 23 <--> 67(tail) -->
    }
}