双链表添加和删除方法

Question

这是我得到的代码。我必须添加代码的添加和删除方法。我已经尝试这样做了一段时间，但是我无法使其通过JUNIT测试。对此，我将不胜感激。祝你有美好的一天！（在第二个代码块中，我尝试添加方法）

包ss.ss.week4;

公共类DoublyLinkedList {

private int size;
private Node head;

/**
 * @ensures the list is empty (size is 0)
 */
public DoublyLinkedList() {
    size = 0;
    head = new Node(null);
    head.next = head;
    head.previous = head;
}

/**
 * Inserts a new Element at a given index in the list.
 * @requires element is not null
 * @requires the index to be within the bounds of the list
 * @ensures the size of the list to increase by one
 * @ensures the Element in the list at index index to be element
 * @param index The index at which to insert the Element
 * @param element The element to add
 */
public void add(int index, Element element) {
    // TODO: implement, see exercise P-4.4

    }


/**
 * Removes an element at a given index
 * @requires the index to be within the bounds of the list
 * @ensures the size of the list to decrease by one
 * @param index the index to remove the element at 
 */
public void remove(int index) {
    // TODO: implement, see exercise P-4.4
}

/**
 * @requires the index to be within the bounds of the list
 */
public Element get(int index) {
    Node p = getNode(index);
    return p.element;
}

/**
 * The node containing the element with the specified index.
 * The head node if the specified index is -1.
 * @requires i to be between -1 and the size of the list
 */
public Node getNode(int i) {
    Node p = head;
    int pos = -1;
    while (pos < i) {
        p = p.next;
        pos = pos + 1;
    }
    return p;
}

public int size() {
    return this.size;
}
public class Node {
    public Node(Element element) {
        this.element = element;
        this.next = null;
        this.previous = null;
    }

    private Element element;
    public Node next;
    public Node previous;

    public Element getElement() {
        return element;
    }
}

}

Node newNode = new Node(element);
    if (index == 0) {
        DoublyLinkedList<Element>.Node first = null;
        newNode.next = first;
        first = newNode;
    } else {
        Node p = getNode(index - 1);
        newNode= p.next;
        p.next = newNode;
    }
    this.size++;