删除 AnyType 链表尾部

Question

我编写了一个节点类，其“数据”字段旨在保存任何类型的数据，而链表类旨在保存具有任何类型数据的节点。我正在实现一个删除重复项（“deleteDuplicates”）的类，发现自己无法将链表的尾部设置为空（“删除尾部”），我觉得这是因为我试图将一个类设置为空.我不明白什么？我希望有人能纠正我的想法。具体看我在方法 deleteDuplicates 中的第 116 行注释掉的代码 TODO。

import java.io.*;

// A node class whose 'data' field is designed to hold any type of data.

class node<AnyType> {

    AnyType data;
    node<AnyType> next;

    // Constructor; sets this object's 'data' field to 'data'.
    node(AnyType data) {
        this.data = data;
    }
}

    // A linked list class designed to hold nodes with any type of data.
public class LinkedList<AnyType> {

    // Notice that when you create a LinkedList object (in main(), for example),
    // you tell it what kind of data it'll be holding. The LinkedList class
    // needs to pass that information on to the node class, as well. That's
    // what's happening here.
    private node<AnyType> head, tail;

    // insert at the tail of the list
    void insert(AnyType data) {
        // if the list is empty, set 'head' and 'tail' to the new node
        if (head == null) {
            head = tail = new node<AnyType>(data);
        }
        // otherwise, append the new node to the end of the list and move the
        // tail reference forward
        else {
            tail.next = new node<AnyType>(data);
            tail = tail.next;
        }
    }

    // insert at the head of the list
    void headInsert(AnyType data) {
        // first, create the node to be inserted
        node<AnyType> YouCanJustMakeANewNode = new node<AnyType>(data);

        // insert it at the beginning of the list
        YouCanJustMakeANewNode.next = head;
        head = YouCanJustMakeANewNode;

        // if the list was empty before adding this node, 'head' AND 'tail'
        // need to reference this new node
        if (tail == null)
            tail = YouCanJustMakeANewNode;
    }

    // print the contents of the linked list
    void printList() {
        for (node<AnyType> temp = head; temp != null; temp = temp.next)
            System.out.print(temp.data + " ");
        System.out.println();
    }

    // Remove the head of the list (and return its 'data' value).
    AnyType removeHead() {
        // if the list is empty, signify that by returning null
        if (head == null)
            return null;

        // Store the data from the head, then move the head reference forward.
        // Java will take care of the memory management when it realizes there
        // are no references to the old head anymore.
        AnyType temp = head.data;
        head = head.next;

        // If the list is now empty (i.e., if the node we just removed was the
        // only node in the list), update the tail reference, too!
        if (head == null)
            tail = null;

        // Return the value from the old head node.
        return temp;
    }

    node<AnyType> deleteNode(node<AnyType> data)
    {
        node<AnyType> helper = head;

        if( helper.equals(data) )
        {
            return head.next;
        }

        while( helper.next != null )
        {
            if( helper.next.equals(data) )
            {
                helper.next = helper.next.next;
                return helper;
            }
            helper = helper.next;
        }
        return helper;
    }

    void deleteDuplicates( LinkedList<Integer> L1 )
    {   
        for (node<Integer> temp = L1.head; temp != null; temp = temp.next)
        {
            for (node<Integer> helper = temp; helper.next != null; helper = helper.next)
            {
                //start at helper.next so that temp doesn't delete it's self
                if( temp.data == helper.next.data && helper.next.next != null )
                {
                        helper.next = helper.next.next; 
                }
                /* TODO: DELETE TAIL
                //can't seem to figure out how to delete the tail
                if( temp.data == helper.next.data && helper.next.next == null )
                {
                        //helper.next = null;
                }
                */

            }
        }
    }



    // returns true if the list is empty, false otherwise
    boolean isEmpty() {
        return (head == null);
    }

    public static void main(String [] args) {

        // create a new linked list that holds integers
        LinkedList<Integer> L1 = new LinkedList<Integer>();
/*      
        for (int i = 0; i < 10; i++)
        {
            // this inserts random values on the range [1, 100]
            int SomeRandomJunk = (int)(Math.random() * 100) + 1;
            System.out.println("Inserting " + SomeRandomJunk);
            L1.insert(SomeRandomJunk);
        }
*/      
        //8,24,15,15,9,9,25,9
        L1.insert(8);
        L1.insert(24);
        L1.insert(15);
        L1.insert(15);
        L1.insert(9);
        L1.insert(9);
        L1.insert(25);
        L1.insert(9);
        //L1.insert(9);
        //L1.insert(9);

        // print the list to verify everything got in there correctly
        System.out.println("Printing integer linked list");
        L1.printList();
        System.out.println("Printing DEL-repeaded LL");
        L1.deleteDuplicates(L1);
        L1.printList();


        // create another linked list (this time, one that holds strings)
        LinkedList<String> L2 = new LinkedList<String>();

        L2.insert("Llamas");
        L2.insert("eat");
        L2.insert("very sexy");
        L2.insert("critical thinking");
        L2.insert("Paper clips annd now I'm ");
        L2.insert("daydreaming");

        // print the new list to verify everything got in there correctly
        while (!L2.isEmpty())
            System.out.print(L2.removeHead() + " ");
        System.out.println();

        // print the old list just to verify that there weren't any static
        // problems that messed things up when we created L2
        L1.printList();
    }
}

Answer 1

改成这样就可以了：

        for (node<Integer> helper = temp; helper != null && helper.next != null; helper = helper.next)
        {
            //start at helper.next so that temp doesn't delete it's self
            if( temp.data == helper.next.data && helper.next.next != null )
            {
                helper.next = helper.next.next;
            }
            //can't seem to figure out how to delete the tail
            if( temp.data == helper.next.data && helper.next.next == null )
            {
                    helper.next = null;
            }
        }

即在循环条件中代替：

helper.next != null

你还需要一个条件：

helper != null && helper.next != null

这是因为在你设置helper.next = null;之后， 循环向前移动并执行helper = helper.next， 然后在下一次迭代中循环条件helper.next != null 抛出 NullPointerException 因为helper 是null。