为什么我的双链表删除以前的链接

Question

我知道这个话题已经被击败了，但我真的在努力实现这两个添加方法到一个链表。addFirst和addLast在自己调用时都可以工作，但是当我调用addFirst("foo")和addLast("bar")时，add last会删除以前添加到列表中的任何内容。Add first应该是在列表的开头添加一项，Add last应该是在列表的末尾添加一项。

import java.util.Iterator;
import java.util.NoSuchElementException;
public class Deque<Item> implements Iterable<Item> {
private int N; 
private Node first;
private Node last;
//create linked list
private class Node
{
    String item;
    Node next;
    Node previous;
}
public Deque()      // construct an empty deque
{
    N = 2; 
    first = new Node();
    last = new Node();
    //link together first and last node;
    first.next = last;
    last.previous = first; 
    last.item = "Last";
    first.item = "First";

}
public boolean isEmpty()                 // is the deque empty?
{
    return first == null;
}
public int size()                        // return the number of items on the deque
{
    return N;
}
public void addFirst(Item item)          // insert the item at the front
{
    Node nextElement = new Node();
    nextElement.item = (String)item;
    nextElement.next = first.next;
    nextElement.previous = first;
    first.next = nextElement;
    N++;
}
public void addLast(Item item)           // insert the item at the end
{
    Node newLast = new Node();
    newLast.item = (String)item;
    newLast.next = last;
    newLast.previous = last.previous;
    last.previous.next = newLast;
    last.previous = newLast;
    N++;
}
public void printList()
{
    Node print = first;
    for (int i = 0; i < N; i++)
    {
        System.out.print(print.item);
        print = print.next;
    }
    System.out.println("");
}

Answer 1

看来你把自己弄糊涂了。一般来说，如果你在做something.next.next或类似的事情，你的头脑中应该会发出警告。您还可以提供一个构造函数，该构造函数可以接受项而不是方法中的加法语句。

public void addLast(Item item)           // insert the item at the end
{
    Node newLast = new Node();
    newLast.item = (String)item;
    if (isEmpty()) {
        first = newLast;
    } else {
        last.next = newLast;
        newLast.previous = last;
    }
    last = newLast;
    N++;
}

就addFirst而言，所以你不会无意中得到不好的建议，它会像这样…

public void addFirst(Item item) {
    Node newFirst = new Node();
    newFirst.item = (String)item;
    if (isEmpty()) {
        last = newFirst;
    } else {
        first.previous = newFirst;
    }
    newFirst.next = first;
    first = newFirst;
    N++;
}

Answer 2

addfirst方法缺少更新一个指针

    public void addFirst(Item item)          // insert the item at the front
{
    Node nextElement = new Node();
    nextElement.item = (String)item;
    nextElement.next = first.next;
    nextElement.previous = first;
    first.next.previous = nextElement; //ADDED HERE
    first.next = nextElement;
    N++;
}

Answer 3

我想这个问题可以用一个简单的链接来回答——你在重新发明轮子，总是一个坏主意，无论你的目标是什么教育目的。

使用Deque接口