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我正在为我为作业设计的课程寻求帮助。它将游戏分数添加到链接列表中，并从最高到最低列出它们。分数的最大数量是 10。我几乎可以工作，但我无法弄清楚。我添加第一个分数并且它有效，然后如果我添加第二个分数，它仅在该分数高于第一个时才有效。如果没有，它会抛出一个java.lang.NullPointerException. 有人可以看看我的insert(String name, int score)方法并让我知道问题所在吗？

public class GamerList {

    /**
     * The node class stores a list element and a reference to the next node.
     * @author johnmckillip
     *
     */

    private class Node {

        String name;
        int score;
        Node next;

        /**
         * Constructor.
         * @param val The element to store in the node.
         * @param n The reference to the successor node.
         */
        Node(String val1, int val2, Node n) {

            name = val1;
            score = val2;
            next = n;
        }

        /**
         * Constructor.
         * @param val The element to store in the node.
         */
        Node(String val1, int val2) {

            this(val1, val2, null);
        }
    }

    private Node head;
    private Node tail;

    /**
     * Constructor.
     */

    public GamerList() {

        head = null;
        tail = null;
    }

    /**
     * The isEmpty method checks to see if the list is empty.
     * @return true if the list is empty, false otherwise.
     */
    public boolean isEmpty() {

        return head == null;
    }

    /**
     * The size method returns the length of the list.
     * @return The number of elements in the list.
     */
    public int size() {

        int count = 0;
        Node p = head;

        while(p != null) {

            count++;
            p = p.next;
        }

        return count;
    }

    public void insert(String name, int score) {

        Node node = new Node(name, score);

        if(isEmpty()) {

            head = node;
            tail = node;
        }

        else if(head.score <= node.score) {

            node.next = head;
            head = node;
        }

        else {

            Node frontPtr = head.next;
            Node backPtr = head;

            while(frontPtr.score > node.score && frontPtr.next != null) {

                backPtr = backPtr.next;
                frontPtr = frontPtr.next;
            }

            if(frontPtr != null && frontPtr.score <= node.score) {

                backPtr.next = node;
                node.next = frontPtr;
            }

            else {

                frontPtr.next = node;
                tail = node;
            }
        }

        if(size() > 10) {

            Node currentPtr = head;

            while(currentPtr.next != tail) {

                currentPtr = currentPtr.next;
            }

            tail = currentPtr;
            currentPtr.next = null;
        }
    }

    public void printList() {

        Node temp = head;

        while(temp != null) {

            System.out.print(temp.name + " " + temp.score + " ");
            System.out.println("");
            temp = temp.next;
        }
    }

}

这是我要测试的课程GamerList：

公共类 TestGamerList {

/**
 * @param args
 */
public static void main(String[] args) {


    GamerList list1 = new GamerList();

    list1.insert("Fry", 89);
    list1.insert("Bender", 25);
    list1.insert("Leela", 90);
    list1.insert("Zoidburg", 23);
    list1.insert("Amy", 34);
    list1.insert("Hermes", 96);
    list1.insert("Zapp",123);
    list1.insert("Nibbler", 56);
    list1.insert("Calculon", 12);
    list1.insert("Hypnotoad", 189);
    list1.insert("Lrrr", 5);
    list1.insert("Scruffy", 28);

    System.out.println("Top 10 Scores: ");
    list1.printList();
}

}

4

2 回答 2

2

看起来你没有设置head's next。这是一个问题。第二个是，即使你这样做，你也会进入无限循环，因为你错误地完成了插入逻辑。我已经对您insert()进行了一些更改以使其正常工作，但这仍然缺乏优雅并且远非有效实施。例如，在您运行 10 个元素后的每次插入，size()这会使您的代码复杂性增加约 10 倍。N = size(). 如果您真的想这样做，请创建size一个变量并在每个insert(). 无论如何，编辑代码：

public class GamerList {

    private class Node {

        String name;
        int score;
        Node next;

        Node(String val1, int val2, Node n) {

            name = val1;
            score = val2;
            next = n;
        }

        Node(String val1, int val2) {

            this(val1, val2, null);
        }
    }

    private Node head;
    private Node tail;

    /**
     * Constructor.
     */

    public GamerList() {

        head = null;
        tail = null;
    }

    /**
     * The isEmpty method checks to see if the list is empty.
     * @return true if the list is empty, false otherwise.
     */
    public boolean isEmpty() {

        return head == null;
    }

    /**
     * The size method returns the length of the list.
     * @return The number of elements in the list.
     */
    public int size() {

        int count = 0;
        Node p = head;

        while(p != null) {

            count++;
            p = p.next;
        }

        return count;
    }

    public void insert(String name, int score) {

        Node node = new Node(name, score);

        if(isEmpty()) {

            head = node;
            head.next = tail;
        }
        else if(head.score <= node.score) {

            node.next = head;
            head = node;
        }

        else {
            Node beforeNode = head;
            while(beforeNode.score > node.score && beforeNode.next != null) {
                beforeNode = beforeNode.next;
            }
            node.next = beforeNode.next;
            beforeNode.next = node;
        }

        if(size() > 10) {

            Node currentPtr = head;

            for (int i = 0; i < 9; i++) {
                currentPtr = currentPtr.next;
            }
            currentPtr.next = null;
        }
    }

    public void printList() {

        Node temp = head;

        while(temp != null) {

            System.out.print(temp.name + " " + temp.score + " ");
            System.out.println("");
            temp = temp.next;
        }
    }

}
于 2013-09-28T20:08:04.027 回答
-1

没有堆栈跟踪很复杂。但可能错误在这里

while(frontPtr.score > node.score && frontPtr.next != null)

因为 frontPtr 为空。

添加一个检查

if (frontPtr!=null)
    while(frontPtr.score > node.score && frontPtr.next != null)
于 2013-09-28T19:40:24.667 回答