1. ArrayList的缺陷

上节课已经熟悉了ArrayList的使用，并且进行了简单模拟实现。通过源码知道，ArrayList底层使用数组来存储元素：

public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
  // ...
  // 默认容量是10
  private static final int DEFAULT_CAPACITY = 10;
  //...
  // 数组：用来存储元素
  transient Object[] elementData; // non-private to simplify nested class access
  // 有效元素个数
  private int size;
  public ArrayList(int initialCapacity) {
  if (initialCapacity > 0) {
  this.elementData = new Object[initialCapacity];
  } else if (initialCapacity == 0) {
  this.elementData = EMPTY_ELEMENTDATA;
  } else {
  throw new IllegalArgumentException("Illegal Capacity: "+
  initialCapacity);
  }
  }
//
}

由于其底层是一段连续空间，当在ArrayList任意位置插入或者删除元素时，就需要将后序元素整体往前或者往后搬移，时间复杂度为O(n)，效率比较低，因此ArrayList不适合做任意位置插入和删除比较多的场景。因此：java集合中又引入了LinkedList，即链表结构。

2. 链表

2.1 链表的概念及结构

链表是一种物理存储结构上非连续存储结构，数据元素的逻辑顺序是通过链表中的引用链接次序实现的。

实际中链表的结构非常多样，以下情况组合起来就有8种链表结构：

单向或者双向
带头或者不带头
循环或者非循环

虽然有这么多的链表的结构，但是我们重点掌握两种:

无头单向非循环链表：结构简单，一般不会单独用来存数据。实际中更多是作为其他数据结构的子结构，如哈希桶、图的邻接表等等。另外这种结构在笔试面试中出现很多

无头双向链表：在Java的集合框架

库中LinkedList底层实现就是无头双向循环链表

2.2 链表的实现

1.链表的功能

package mysingleList;
public interface IList {
    void addFirst(int data);
    //尾插法
    void addLast(int data);
    //任意位置插入,第一个数据节点为0号下标
    void addIndex(int index,int data);
    //查找是否包含关键字key是否在单链表当中
    boolean contains(int key);
    //删除第一次出现关键字为key的节点
    void remove(int key);
    //删除所有值为key的节点
    void removeAllKey(int key);
    //得到单链表的长度
    int size();
    void clear();
    void display();
}

2.初始化链表

public class MySingleList implements IList{
    static class ListNode{
        public int val;
        public ListNode next;
        public ListNode(int val){
            this.val = val;
        }
    }
    public ListNode head;
    public void createList(){
        ListNode node1 = new ListNode(12);
        ListNode node2 = new ListNode(23);
        ListNode node3 = new ListNode(34);
        ListNode node4 = new ListNode(45);
        ListNode node5 = new ListNode(56);
        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node5;
        this.head = node1;
    }

开辟了内存空间

使每个node的next域指向下一个节点的地址，连接成链表

head指向第一个节点的地址

3.实现功能接口

3.1头插添加元素

 public void addFirst(int data) {
        ListNode node = new ListNode(data);
        if(this.head == null){
            this.head = node;
        }
        else {
            node.next = this.head;
            this.head = node;
        }
    }

对 node.next = this.head;

this.head = node;

进行解释，node的next域指向下一个节点的地址

head继续为头节点

3.2尾插法添加新元素

public void addLast(int data) {
        ListNode node = new ListNode(data);
        ListNode cur = head;
        if (this.head == null){
            this.head = node;
        }
        else {
            while(cur.next != null){
                cur = cur.next;
            }
            cur.next = node;
        }
    }

找到最后一个元素cur，cur的next指向要插入元素的地址

3.3找到下标的前驱节点

 private ListNode searchPrev(int index){
            ListNode cur = this.head;
            int count = 0;
            while(count != index-1){
                cur = cur.next;
                count++;
            }
            return cur;
        }

3.4指定位置插入元素

public void addIndex(int index, int data) {
    //判断index的位置是否合法
        if(index < 0 || index >size()){
            return;
        }
        //插入到第一个节点位置
        if(index == 0){
            addFirst(data);
        }
        //插入到最后一个节点的位置
        if (index == size()){
            addLast(data);
        }
        //中间位置
        else {
            ListNode node = new ListNode(data);
            ListNode cur = searchPrev(index);
            node.next = cur.next;
            cur.next = node;
        }
    }

3.5指定元素是否存在

public boolean contains(int key) {
        ListNode cur = this.head;
        while(cur != null){
            if(cur.val == key){
                return true;
            }
        }
        return false;
    }

遍历一遍链表寻找是否有key元素

3.6找到指定元素的前驱节点

private ListNode findPrev(int key){
        ListNode cur = this.head;
        while(cur.next != null){
            if (cur.next.val == key){
                return cur;
            }
            cur = cur.next;
        }
        return null;
    }

3.7删除指定节点

public void remove(int key) {
        if (this.head == null){
            System.out.println("没有节点，无法删除");
            return;
        }
        //指定元素在头节点
        if (this.head.val == key){
            this.head = this.head.next;
        }
        else {
            ListNode cur = findPrev(key);
            //没有找到指定元素
            if (cur == null){
                System.out.println("没有找到要删除的节点");
                return;
            }
            //找到了指定元素
           ListNode del = cur.next;
            cur.next = del.next;
        }
    }

3.8删除所有元素为key的节点

public void removeAllKey(int key) {
        if(this.head == null){
            return;
        }
        ListNode prev = this.head;
        ListNode cur = this.head.next;
        while(cur != null){
            if(cur.val == key){
                prev.next = cur.next;
                cur = cur.next;
            }
            else {
                prev = cur;
                cur = cur.next;
            }
        }
        //删除的节点为头节点
        if(this.head.val == key){
            this.head = this.head.next;
        }
    }

3.9链表的长度

public int size() {
        ListNode cur = this.head;
        int count = 0;
        while(cur != null) {
            count++;
            cur = cur.next;
        }
        return count;
    }

3.9清空链表

public void clear() {
        ListNode cur = this.head;
        while(cur != null){
            ListNode curNext = cur.next;
            cur.next = null;
            cur = curNext;
        }
        head = null;
    }

完整代码

package mysingleList;
public class MySingleList implements IList{
    static class ListNode{
        public int val;
        public ListNode next;
        public ListNode(int val){
            this.val = val;
        }
    }
    public ListNode head;
    public void createList(){
        ListNode node1 = new ListNode(12);
        ListNode node2 = new ListNode(23);
        ListNode node3 = new ListNode(34);
        ListNode node4 = new ListNode(45);
        ListNode node5 = new ListNode(56);
        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node5;
        this.head = node1;
    }
    @Override
    public void addFirst(int data) {
        ListNode node = new ListNode(data);
        if(this.head == null){
            this.head = node;
        }
        else {
            node.next = this.head;
            this.head = node;
        }
    }
    @Override
    public void addLast(int data) {
        ListNode node = new ListNode(data);
        ListNode cur = head;
        if (this.head == null){
            this.head = node;
        }
        else {
            while(cur.next != null){
                cur = cur.next;
            }
            cur.next = node;
        }
    }
    @Override
    public void addIndex(int index, int data) {
        if(index < 0 || index >size()){
            return;
        }
        if(index == 0){
            addFirst(data);
        }
        if (index == size()){
            addLast(data);
        }
        else {
            ListNode node = new ListNode(data);
            ListNode cur = searchPrev(index);
            node.next = cur.next;
            cur.next = node;
        }
    }
        private ListNode searchPrev(int index){
            ListNode cur = this.head;
            int count = 0;
            while(count != index-1){
                cur = cur.next;
                count++;
            }
            return cur;
        }
    @Override
    public boolean contains(int key) {
        ListNode cur = this.head;
        while(cur != null){
            if(cur.val == key){
                return true;
            }
        }
        return false;
    }
    @Override
    public void remove(int key) {
        if (this.head == null){
            System.out.println("没有节点，无法删除");
            return;
        }
        if (this.head.val == key){
            this.head = this.head.next;
        }
        else {
            ListNode cur = findPrev(key);
            if (cur == null){
                System.out.println("没有找到要删除的节点");
                return;
            }
           ListNode del = cur.next;
            cur.next = del.next;
        }
    }
    private ListNode findPrev(int key){
        ListNode cur = this.head;
        while(cur.next != null){
            if (cur.next.val == key){
                return cur;
            }
            cur = cur.next;
        }
        return null;
    }
    @Override
    public void removeAllKey(int key) {
        if(this.head == null){
            return;
        }
        ListNode prev = this.head;
        ListNode cur = this.head.next;
        while(cur != null){
            if(cur.val == key){
                prev.next = cur.next;
                cur = cur.next;
            }
            else {
                prev = cur;
                cur = cur.next;
            }
        }
        if(this.head.val == key){
            this.head = this.head.next;
        }
    }
    @Override
    public int size() {
        ListNode cur = this.head;
        int count = 0;
        while(cur != null) {
            count++;
            cur = cur.next;
        }
        return count;
    }
    @Override
    public void clear() {
        ListNode cur = this.head;
        while(cur != null){
            ListNode curNext = cur.next;
            cur.next = null;
            cur = curNext;
        }
        head = null;
    }
    @Override
    public void display() {
        ListNode cur = this.head;
        while (cur != null){
            System.out.print(cur.val+" ");
            cur = cur.next;
        }
        System.out.println();
    }
}

【数据结构】LinkedList与链表