数据结构——双向链表（Java实现）

      费了将近一天时间，终于实现了一个双向链表，支持正向和逆向的输出；

      当实现逆向输出时有一问题一直困扰着我，就是删除某个指定元素后，正向输出显示已删除该元素，逆向输出显示没有删除该元素，百思不得姐啊，也知道问题出在deleteKey(long dd)方法中，但是具体位置找不到，后来逐步试验，略得蛛丝马迹，发现删除表头元素和表尾元素后正向输出和逆向输出是一致的，而删除其他地方元素两者输出不一致，最后将问题锁定在“当前指针指在一般位置时的代码处”，恩！问题果然在这里，写引用时少写了一个，逻辑上出了问题，导致没能正常删掉当前数据项，下面就剖析一下我的错误！

      没有执行删除代码前，对于双向链表来说一般位置上的数据项（非表头和表尾）前后的引用形式如下图所示：

      执行完我最初写的代码后，代码如下，引用变成如下形式：

current.previous.next = current.next;current.next = current.previous;

      执行完代码后：

      所以才会出现正向输出时显示删除了该数据项，逆向输出时没有删除该数据项。

      经分析，是引用的形式写错了，代码应该写成如下形式：

current.previous.next = current.next;current.next.previous = current.previous;

      执行完毕，该数据项被删除了，变成一座孤岛，没有人与他连接，被“孤立”起来，等待jvm中回收机制去回收，形式如下：

      正向输出和逆向输出终于保持一致了！

      完整代码如下：

class Link{public long dData;public Link next;public Link previous;//-------------------public Link(long d){dData = d;}//-------------------public void displayLink()  //display this link;{System.out.print(dData + " ");}}//////////////////////////////////////////class DoublyLinkedList{private Link first;private Link last;//----------------------public DoublyLinkedList(){first = null;last = null;}//----------------------public boolean isEmpty(){return first == null;}//---------------------public void insertFirst(long dd){Link newLink = new Link(dd);if(isEmpty()){last = newLink;}else{first.previous = newLink;}newLink.next = first;first = newLink;/*  This part is designed by me, I may right! if(isEmpty()){first = newLink;last = newLink;}else{first.previous = newLink;newLink.next = first;first = newLink;}*/}//--------------------public void insertLast(long dd) //by me!{Link newLink = new Link(dd);if(isEmpty()){first = newLink;last = newLink;}else{last.next = newLink;newLink.previous = last;last = newLink;}}//------------------------------------------by me!public Link deleteFirst(){Link temp = first;    // what I designed support emtpy listif(first == null) // an empty list{System.out.println("Your list is an empty one, nothing to delete");return null;}if(first.next == null) //just one item is the list{last = null;first = null;}else{first.next.previous = null;first = first.next;}return temp;}//-------------------------------------------public Link deleteLast(){Link temp = last;//save the last one !  what I designed support emtpy listif(last == null)//an empty list{System.out.println("Your list is an empty one, nothing to delete");return null;}if(last.previous == null){last = null;first = null;}else{last.previous.next = null;last = last.previous;}return temp;}//-------------------------------------------public void insertAfter(long key, long dd)  ////by me! make sure the "key" is already in your list;{Link current = first;Link temp = null;if(current==null) //the list is an empty one!{System.out.println("Your list is am empty one, please insert something first");return;}    else    {    while(current.dData!=key && current!=last)    {    current = current.next;    }    }    Link newLink = new Link(dd);    if(current==first)    {    temp = current.next;//these five lins codes can be written just once!    current.next = newLink;    newLink.next = temp;    temp.previous = newLink;    newLink.previous = current;    }    else if(current==last)    {    if (current.dData!=key) {    System.out.println("Cannot find the item according the key you give!");    return ;    }    newLink.previous = current;    current.next = newLink;    last = newLink;    }    else    {    temp = current.next;    current.next = newLink;    newLink.next = temp;    temp.previous = newLink;    newLink.previous = current;    }}//-------------------------------------------public Link deleteKey(long key){Link current = first;Link currentNext = null;if(first==null){System.out.println("You have nothing to delete, your list is an empty one!");return null;}else{while(current.dData!=key)    {    current = current.next;    if(current==null)    {    System.out.println("You don't have the item in your list");    return null;    }    }}if(current==first)  //support to delete the first item{current.next.previous = null;first = current.next;return current;}if(current==last){current.previous.next = null;last = current.previous;return current;}else{current.previous.next = current.next;current.next.previous = current.previous;//what I first wrote is :current.next = current.previous;return current;}}//--------------------------------------------public void displayF(){System.out.print("List (first-->last): ");Link current = first;if(current==null){System.out.print("empty list");}while(current!=null) //assume non-empty list{current.displayLink();current = current.next;}System.out.println();}//---------------------------------------------public void displayBackword(){Link current = last;System.out.print("List (last-->first): ");if(current==null){System.out.print("empty list");}while(current!=null){current.displayLink();current = current.previous;}System.out.println();}}//-----------------------------------------------------public class DoublyLinkedApp{public static void main(String[] args){DoublyLinkedList list = new DoublyLinkedList();list.insertFirst(80);list.insertFirst(70);list.insertFirst(60);list.insertLast(21);list.insertLast(22);list.insertLast(23);list.displayForward();list.displayBackword();System.out.println();//leave a line to see the result clearly!    System.out.println("when I insert 564 after the item 12 , the list becomes: ");list.insertAfter(70,564);list.displayForward();list.displayBackword();System.out.println();//leave a line to see the result clearly!        System.out.println("when I delete the item 21 and 22 form the list, it becomes: ");        list.deleteKey(21);        list.deleteKey(22);        list.displayForward();        list.displayBackword();   }}

      程序运行结果如下：

本博文代码托管至Github，来Github找我