Question

我必须使用自己的构造函数来实现双向循环链表，我已经完成了很多工作，但无法弄清remove方法为什么不起作用。

我已经做了大量研究，但是我很难找到符合我需要的东西。问题是我没有永久的头和尾指针，就像通常在双链表中一样，但是必须使用“标头”作为起点和终点。

带有标头元素的构造函数

public MyDoubleLinkedList() {
        header = new DEntry(0, null, null);
        header.next = header;
        header.previous = header;
        size = 0;
    }

listEntrys的内部类

    class DEntry {
        /** the data element represented by this entry */
        private final int data;

        /** reference to the previous element in the list */
        private DEntry previous;

        /** reference to the next element in the list */
        private DEntry next;

        /**
         * @param data     the data object this entry represents
         * @param previous reference to the previous element in the list
         * @param next     reference to the next element in the list
         */
        public DEntry(int data, DEntry previous, DEntry next) {
            this.data = data;
            this.previous = previous;
            this.next = next;
        }
    }

添加到列表中的方法：

    /**
     * Adds a new element into the list at the position specified
     * 
     * @param position the 0 based position at which to add the passed value
     * @param value    the value to add
     * @return 0 if adding was successful, -1 if not
     */
    public int add(int position, int value) {
        // TODO: please put your code here
        DEntry listEntry = new DEntry(value, null, null);

        DEntry temp = header;
        int i = 0;

        if (position < 0 || position > size) {
            return -1;
        }

        if (position == 0) {
            temp = header;

        } else {
            while (i < position) {
                temp = temp.next;
                i++;
            }
        }

        listEntry.next = temp.next;
        listEntry.previous = temp.next;
        temp.next = listEntry;
        temp.next.previous = listEntry.next;
        size++;

        return 0;
    }

从列表中删除的方法

    /**
     * Removes an element at the position specified from the list
     * 
     * @param position the 0 based position of the value to remove
     * @return value of the removed entry if removing was successful, -1 if not
     */
    public int remove(int position) {
        // TODO: please put your code here

    DEntry toBeDeleted = header;

        if(position < 0 || position > size) {
            return -1;
        }
        if(getEntry(position) == null) {
            return -1;
        } else {
        toBeDeleted = getEntry(position);
        }

        int dataOfDeletedNode = toBeDeleted.data;

        if(position == 0) {
            header.previous.next = toBeDeleted.next;
            header.next.previous = toBeDeleted.previous;
        } else if(position == size){
            toBeDeleted.previous.next = header.next;
            toBeDeleted.next.previous = toBeDeleted.previous;
        } else {
            toBeDeleted.previous.next = toBeDeleted.next;
            toBeDeleted.next.previous = toBeDeleted.previous;
        }



        size--;
        System.out.println(dataOfDeletedNode);
        return dataOfDeletedNode;
    }

如果我运行代码

list.add(0, 10);
list.add(1, 20);
list.add(0, 30);
remove(1); // 10 should be deleted

我只有20个，而不是30个。

Answer 1

看来您的主要问题来源是add方法。实际上，在代码中链接新节点存在很大的问题，这是我通过阅读代码发现的唯一问题。因此，您的add方法应如下所示：

public int add(int position, int value) {
    DEntry listEntry = new DEntry(value, null, null);

    DEntry temp = header;

    if (position < 0 || position > size) {
        return -1;
    }

    if (position == 0) {
        temp = header;

    } else {
        int i = 0;
        while (i < position) {
            temp = temp.next;
            i++;
        }
    }

    listEntry.next = temp.next;
    listEntry.previous = temp;
    temp.next = listEntry;
    size++;

    return 0;
}

listEntry将是临时节点之后的下一个节点。然后，它的前一个指针应指向温度。
在临时节点之后放置新节点不需要在临时节点的先前链接中进行任何更改。因此，您在代码中的最后一个链接是有问题的。

Answer 2

我可以同时解决这个问题。确实是我的添加方法阻止了我的删除方法正常工作。

部分出错是我的while循环在（i

这是我在add方法中想到的，一切都很好。

public int add(int position, int value) {

    // Creates a new listEntry 
    DEntry listEntry = new DEntry(value, null, null);
    DEntry temp = header;

    int i = 0;

    if (position < 0 || position > size) {
        return -1;
    }

        while (i <= position) {
            temp = temp.next;
            i++;
        }

    // setting the elements neighbours
    listEntry.next = temp;
    listEntry.previous = temp.previous;

    // placing the new element between last and next
    temp.previous.next = listEntry;
    temp.previous = listEntry;

    // places the new entry in the list
    temp = listEntry;
    size++;

    return 0;
}

如何正确地从双向循环链表中删除元素？

2 个答案: