java - 传输 DLList 的字符串时出现问题

Question

我应该创建一个名为“Backwards”的方法，该方法将列表从尾部横向到头部，但是当我运行代码时，它出现说(第 88 行)它找不到光标 = cusor.prev;象征。我需要在循环中再次设置上一个链接吗？感谢您的帮助

import java.util.*;

public class DLList<E>{
    //data
    private DLLNode<E> head;
    private DLLNode<E> tail;
    private DLLNode<E> prev;
    private DLLNode<E> next;

    //constructor(s)
    public DLList()
    {
        head = tail = null;
    }

    // ------------ methods

    //addFirst - adds a node with theData to the front of the list
    public void addFirst(E theData)
    {
        //case where the list was empty
        if (head == null)
        {
            head = tail = new DLLNode<E>(theData);
        }
        else
        {
            DLLNode<E> temp = new DLLNode<E>(theData);
            temp.next = head;
            head = temp;
        }
    }    

    // addLast - adds a node with theData to the end of the list
    public void addLast(E theData)
    {
        //case where the list was empty
        if (head == null)
        {
            head = tail = new DLLNode<E>(theData);
        }
        else    //case where there was only 1 OR there are many element(s)
        {
            DLLNode<E> temp = new DLLNode<E>(theData);   //create the new node
            tail.next = temp;  //reset the last link
            tail = temp;   // tail is reset to the new last node
        }
    }

    //toString - returns the DLList as a String
    public String toString()
    {
        String retString = " ";


        //traverse through the whole list (starting at head, until last link is null)
        DLLNode<E> cursor = head;
        while (cursor != null)
        {
            if (cursor == head)
                retString = retString + "" + cursor.data;
            else   //not first element so put in the comma
                retString = retString + ", " + cursor.data;
            cursor = cursor.next;
        }


        return "[" + retString + " ]";
    }


    //backwards - returns the DLList as a String (BACKWARDS)
    public String backwards()
    {

        String retString = " ";


        //traverse through the whole list (starting at tail, until last link is null)
        DLLNode<E> cursor = tail;
        while (cursor != null)
        {
            if (cursor == tail)
                retString = retString + "" + cursor.data;
            else   //not first element so put in the comma
                retString = retString + ", " + cursor.data;
            cursor = cursor.prev;
        }


        return "[" + retString + " ]";
    }

    //recursive toString - this is the method that is called from "outside"
    //and just calls the recursive version
    public String anotherToString()
    {
        return recursiveToString(head);
    }

    //recursive version - calls itself
    private String recursiveToString(DLLNode<E> subList)
    {
        if (subList == null)
            return "";
        else
            return recursiveToString(subList.next) + "  " + subList.data;
    }


    //getFirst - returns the first element on the list (without deleting it)
    public E getFirst()
    {
        if (head == null)    //empty
            throw new NoSuchElementException("can't getFirst from empty list");
        return head.data;
    }

    //getLast - returns the last element on the list (without deleting it)
    public E getLast()
    {
        if (head == null)    //empty
            throw new NoSuchElementException("can't getLast from empty list");
        return tail.data;
    }

    //contains - returns true if what is received is on the list
    public boolean contains(E something)
    {
        DLLNode<E> cursor = head;

        while (cursor != null)
        {
            if (cursor.data.equals(something))
                return true;    // found it so return now!
            cursor = cursor.next;
        }

        return false;   // if we got through the whole loop and didn't return, its not there
    }

    //size - returns the size of the DLList
    public int size()
    {
        int count = 0;
        //traverse through the whole list (starting at head, until last link is null)
        DLLNode<E> cursor = head;
        while (cursor != null)
        {
            count++;
            cursor = cursor.next;
        }    

        return count;
    }

    //add - adds a new element at a given index
    public void add(int index, E elt)
    {
        //is index OK?
        if (index < 0 || index > size())
            throw new IllegalArgumentException("index " + index + " is out of bounds");

        if (index == 0)      //goes at the head
            addFirst(elt);    //call our own method
        else if (index == size())      //goes at the tail
            addLast(elt);     //call our own method
        else     //goes in the middle somewhere
        {
            DLLNode<E> cursor = head;     //set up a cursor
            for (int i=1; i<index; i++)
                cursor = cursor.next;       //move the cursor over index times

            //the cursor should have stopped at the node right before the insertion
            //so just create the new node and then change the link
            DLLNode<E> temp = new DLLNode<E>(elt);
            temp.next = cursor.next;
            cursor.next = temp;
        }
    }


    //removeFirst - removes and returns the first element on the list
    public E removeFirst()
    {
        //case1: list is empty
        if (head == null)
            throw new NoSuchElementException("cannot removeFirst from empty list");

        //case2: list only has 1 element
        else if (head == tail)
        {
            E whatToReturn = head.data;    //keep track of it
            head = tail = null;
            return whatToReturn;
        }

        //case3: list has many element
        else
        {
            E whatToReturn = head.data;    //keep track of it
            head = head.next;    //move head over
            return whatToReturn;
        }
    }

    //removeLast - removes and returns the last element on the list
    public E removeLast()
    {
        //case1: list is empty
        if (head == null)
            throw new NoSuchElementException("cannot removeLast from empty list");

        //case2: list only has 1 element
        else if (head == tail)
        {
            E whatToReturn = head.data;    //keep track of it
            head = tail = null;
            return whatToReturn;
        }

        //case3: list has many elements
        else
        {
            E whatToReturn = tail.data;    //keep track of it

            //cycle through whole list, stopping at the node right BEFORE the tail
            DLLNode<E> cursor = head;
            while (cursor.next != tail)
                cursor = cursor.next;

            //at this point, cursor should point at the node right before tail
            //make changes...
            tail = cursor;
            tail.next = null;

            //return what was the last data
            return whatToReturn;
        }
    }

    //remove - remove and return the first occurrance of an element
    public boolean remove(E doomedElt)
    {
        //if the list is empty, then it obviously can't be removed
        if (head == null)
            return false;

        //if the list has 1 or more elements
        else
        {
            //find it (if it exists)
            DLLNode<E> cursor = head;
            while (cursor != null && !cursor.data.equals(doomedElt))
            {
                cursor = cursor.next;
            }

            //if its got all the way through the list (so it points at null now)
            //then it did not find doomedElt
            if (cursor == null)
                return false;

            //otherwise, we found it and cursor points to it
            else
            {
                //if its the first element to be removed, then call our own method
                if (cursor == head)
                {
                    removeFirst();
                    return true;
                }    

                //if its not the first element, then we have to traverse the list again
                //so we know the element in front of it
                else
                {
                    DLLNode<E> followCursor = head;
                    while (!followCursor.next.equals(cursor))
                        followCursor = followCursor.next;
                    followCursor.next = cursor.next;
                    return true;
                }
            }
        }
    }

    //isEmpty - returns true if it is empty
    public boolean isEmpty()
    {
        return head == null;
    }    


//we need a Node to hold the data - it is its own class
private class DLLNode<E>
{
    //data
    protected E data;                       //protected so it is visible inside the DLList class
    protected DLLNode<E> next;    //(so I won't need .getData, .setData...)

    //constructor(s)
    public DLLNode(E theData)
    {
        this.data = theData;
        next = null;
    }

    //methods
    public String toString()
    {
        return data.toString();
    }
}
 }  //end of DLList class

Answer 1

好吧，如果我们查看 DLLNode，它没有任何名为 prev 的成员。

只需添加一个:

private class DLLNode<E>
{
    //data
    protected E data;                       //protected so it is visible inside the DLList class
    protected DLLNode<E> next;    //(so I won't need .getData, .setData...)
    protected DLLNode<E> prev;
    //constructor(s)
    ...