4.数据结构--链表

1.什么是链表

优点:不需要处理固定容量的问题

缺点:丧失了随机访问的能力

2.数组和链表的对比

3.在链表中添加元素

(1)在链表头添加元素

(2)在链表中间添加元素

(3)链表中添加节点的代码实现

public class LinkedList<E> {
    private class Node{
        public E e;
        public Node next;

        public Node(E e,Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    private Node head;
    int size;

    public LinkedList(){
        head = null;
        size =0;
    }

    //获取链表中的元素个数
    public int getSize(){
        return size;
    }

    //返回链表是否为空
    public boolean isEmpty(){
        return size == 0;
    }

    //在链表头添加新的元素e
    public void addFirst(E e){
//        Node node = new Node(e);
//        node.next = head;
//        head = node;

        head = new Node(e,head);
        size ++;
    }

    //在链表的index位置添加新元素e
    public void add(int index,E e){
        if(index < 0 || index > size)
            throw new IllegalArgumentException("Add failed.Illegal index.");

        if(index == 0)
            addFirst(e);
        else{
            Node prev = head;
            for(int i = 0;i < index -1;i ++)
                prev =prev.next;
//            Node node =new Node(e);
//            node.next = prev.next;
//            prev.next = node;

            prev.next =new Node(e,prev.next);
            size ++;
        }
    }

    //在链表末尾添加新的元素e
    public void addLast(E e){
        add(size,e);
    }
}

4.使用链表的虚拟头结点

public class LinkedList<E> {
    private class Node{
        public E e;
        public Node next;

        public Node(E e,Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    private Node dummyHead;
    private int size;

    public LinkedList(){
        dummyHead = new Node(null,null);
        size =0;
    }

    //获取链表中的元素个数
    public int getSize(){
        return size;
    }

    //返回链表是否为空
    public boolean isEmpty(){
        return size == 0;
    }

    //在链表的index位置添加新元素e
    public void add(int index,E e){
        if(index < 0 || index > size)
            throw new IllegalArgumentException("Add failed.Illegal index.");

        Node prev = dummyHead;
        for(int i = 0;i < index;i ++)
            prev =prev.next;

        prev.next =new Node(e,prev.next);
        size ++;
    }

    //在链表头添加新的元素e
    public void addFirst(E e){
        add(0,e);
    }

    //在链表末尾添加新的元素e
    public void addLast(E e){
        add(size,e);
    }
}

5.链表的遍历，查询和修改

public class LinkedList<E> {
    private class Node{
        public E e;
        public Node next;

        public Node(E e,Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    private Node dummyHead;
    private int size;

    public LinkedList(){
        dummyHead = new Node(null,null);
        size =0;
    }

    //获取链表中的元素个数
    public int getSize(){
        return size;
    }

    //返回链表是否为空
    public boolean isEmpty(){
        return size == 0;
    }

    //在链表的index位置添加新元素e
    public void add(int index,E e){
        if(index < 0 || index > size)
            throw new IllegalArgumentException("Add failed.Illegal index.");

        Node prev = dummyHead;
        for(int i = 0;i < index;i ++)
            prev =prev.next;

        prev.next =new Node(e,prev.next);
        size ++;
    }

    //在链表头添加新的元素e
    public void addFirst(E e){
        add(0,e);
    }

    //在链表末尾添加新的元素e
    public void addLast(E e){
        add(size,e);
    }

    //获得链表的第index位置的元素
    public E get(int index){
        if(index < 0 || index >= size)
            throw new IllegalArgumentException("Get Failed.Illegal index.");

        Node cur = dummyHead.next;
        for(int i = 0;i < index;i ++)
            cur = cur.next;
        return cur.e;
    }

    //获得链表的第一个元素
    public E getFirst(){
        return get(0);
    }

    //获得链表的最后一个元素
    public E getLast(){
        return get(size - 1);
    }

    //修改链表的第index位置的元素e
    public void set(int index,E e){
        if(index < 0 || index >= size)
            throw new IllegalArgumentException("Set failed.Illegal index.");

        Node cur = dummyHead.next;
        for(int i = 0;i <index;i ++)
            cur = cur.next;
        cur.e = e;
    }

    //查找链表中是否有元素e
    public boolean contains(E e){
        Node cur = dummyHead.next;
        while(cur != null){
            if(cur.e.equals(e))
                return true;
            cur = cur.next;
        }
        return false;
    }

    @Override
    public String toString(){
        StringBuilder res =new StringBuilder();
        Node cur =dummyHead.next;
        while(cur != null){
            res.append(cur + "->");
            cur =cur.next;
        }
        res.append("NULL");
        return res.toString();
    }
}

调用链表实例

public class Main {
    public static void main(String[] args) {
        LinkedList<Integer> linkedList = new LinkedList<>();
        for(int i = 0;i < 5; i ++){
            linkedList.addFirst(i);
            System.out.println(linkedList);
        }
        linkedList.add(2,666);
        System.out.println(linkedList);
//        0->NULL
//        1->0->NULL
//        2->1->0->NULL
//        3->2->1->0->NULL
//        4->3->2->1->0->NULL
//        4->3->666->2->1->0->NULL
    }
}

6.链表元素的删除

public class LinkedList<E> {
    private class Node{
        public E e;
        public Node next;

        public Node(E e,Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    private Node dummyHead;
    private int size;

    public LinkedList(){
        dummyHead = new Node(null,null);
        size =0;
    }

    //获取链表中的元素个数
    public int getSize(){
        return size;
    }

    //返回链表是否为空
    public boolean isEmpty(){
        return size == 0;
    }

    //在链表的index位置添加新元素e
    public void add(int index,E e){
        if(index < 0 || index > size)
            throw new IllegalArgumentException("Add failed.Illegal index.");

        Node prev = dummyHead;
        for(int i = 0;i < index;i ++)
            prev =prev.next;

        prev.next =new Node(e,prev.next);
        size ++;
    }

    //在链表头添加新的元素e
    public void addFirst(E e){
        add(0,e);
    }

    //在链表末尾添加新的元素e
    public void addLast(E e){
        add(size,e);
    }

    //获得链表的第index位置的元素
    public E get(int index){
        if(index < 0 || index >= size)
            throw new IllegalArgumentException("Get Failed.Illegal index.");

        Node cur = dummyHead.next;
        for(int i = 0;i < index;i ++)
            cur = cur.next;
        return cur.e;
    }

    //获得链表的第一个元素
    public E getFirst(){
        return get(0);
    }

    //获得链表的最后一个元素
    public E getLast(){
        return get(size - 1);
    }

    //修改链表的第index位置的元素e
    public void set(int index,E e){
        if(index < 0 || index >= size)
            throw new IllegalArgumentException("Set failed.Illegal index.");

        Node cur = dummyHead.next;
        for(int i = 0;i <index;i ++)
            cur = cur.next;
        cur.e = e;
    }

    //查找链表中是否有元素e
    public boolean contains(E e){
        Node cur = dummyHead.next;
        while(cur != null){
            if(cur.e.equals(e))
                return true;
            cur = cur.next;
        }
        return false;
    }

    //从链表中删除index位置的元素,返回删除的元素
    public E remove(int index){
        if(index < 0 || index >=size)
            throw new IllegalArgumentException("Remove failed.Index is illegal");
        Node prev = dummyHead;
        for(int i = 0;i < index; i ++)
            prev = prev.next;
        Node retNode = prev.next;
        prev.next = retNode.next;
        retNode.next = null;

        size --;
        return retNode.e;
    }

    //从链表中删除第一个元素,返回删除的元素
    public E removeFirst(){
        return remove(0);
    }

    //从链表中删除最后一个元素,返回删除的元素
    public E removeLast(){
        return remove(size - 1);
    }

    @Override
    public String toString(){
        StringBuilder res =new StringBuilder();
        Node cur =dummyHead.next;
        while(cur != null){
            res.append(cur + "->");
            cur =cur.next;
        }
        res.append("NULL");
        return res.toString();
    }
}

调用删除链表实例:

public class Main {
    public static void main(String[] args) {
        LinkedList<Integer> linkedList = new LinkedList<>();
        for(int i = 0;i < 5; i ++){
            linkedList.addFirst(i);
            System.out.println(linkedList);
        }
        linkedList.add(2,666);
        System.out.println(linkedList);
//        0->NULL
//        1->0->NULL
//        2->1->0->NULL
//        3->2->1->0->NULL
//        4->3->2->1->0->NULL
//        4->3->666->2->1->0->NULL

        linkedList.remove(2);
        System.out.println(linkedList);

        linkedList.removeFirst();
        System.out.println(linkedList);

        linkedList.removeLast();
        System.out.println(linkedList);
//        0->NULL
//        1->0->NULL
//        2->1->0->NULL
//        3->2->1->0->NULL
//        4->3->2->1->0->NULL
//        4->3->666->2->1->0->NULL
//        4->3->2->1->0->NULL
//        3->2->1->0->NULL
//        3->2->1->NULL
    }
}

7.链表的时间复杂度分析

 

 8.使用链表实现栈

public class LinkedListStack<E> implements Stack<E>{
    private LinkedList<E> list;
    public LinkedListStack(){
        list = new LinkedList<>();
    }

    @Override
    public int getSize(){
        return list.getSize();
    }

    @Override
    public boolean isEmpty(){
        return list.isEmpty();
    }

    @Override
    public void push(E e){
        list.addFirst(e);
    }

    @Override
    public E pop(){
        return list.removeFirst();
    }

    @Override
    public E peek(){
        return list.getFirst();
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Stack: top ");
        res.append(list);
        return res.toString();
    }

    public static void main(String[] args) {
        LinkedListStack<Integer> stack = new LinkedListStack<>();
        for(int i = 0;i < 5;i ++){
            stack.push(i);
            System.out.println(stack);
        }
        stack.pop();
        System.out.println(stack);
//        Stack: top 0->NULL
//        Stack: top 1->0->NULL
//        Stack: top 2->1->0->NULL
//        Stack: top 3->2->1->0->NULL
//        Stack: top 4->3->2->1->0->NULL
//        Stack: top 3->2->1->0->NULL
    }
}

 9.数组栈和链表栈比较

import java.util.Random;

public class Main {//测试使用q运行opCount个enqueue和dequeue操作所需要的时间,单位:秒
    private static double testStack(Stack<Integer> stack,int opCount){
        long startTime = System.nanoTime();
        Random random = new Random();
        for(int i = 0; i < opCount; i++)
            stack.push(random.nextInt(Integer.MAX_VALUE));
        for(int i = 0;i < opCount; i++)
            stack.pop();
        long endTime = System.nanoTime();
        return (endTime - startTime) / 1000000000.0;
    }

    public static void main(String[] args) {
        int opCount = 10000000;
        //数组栈
        ArrayStack<Integer> arrayStack = new ArrayStack<>();
        double time1 = testStack(arrayStack,opCount);
        System.out.println("ArrayStack, time:" + time1 + "s");       // ArrayStack, time:4.220762637s

        //链表栈
        LinkedListStack<Integer> linkedListStack = new LinkedListStack<>();
        double time2 = testStack(linkedListStack,opCount);
        System.out.println("LinkedListStack, time:" + time2 + "s");  // LinkedListStack, time:6.295457665s
    }
}

 10.改进我们的链表来实现队列

public class LinkedListQueue<E> implements Queue<E> {
    private class Node{
        public E e;
        public Node next;

        public Node(E e,Node next){
            this.e = e;
            this.next = next;
        }

        public Node(E e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }

        @Override
        public String toString(){
            return e.toString();
        }
    }

    private Node head , tail;
    private int size;

    public LinkedListQueue(){
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public int getSize(){
        return size;
    }

    @Override
    public boolean isEmpty(){
        return size == 0;
    }

    @Override
    public void enqueue(E e){
        if(tail == null){
            tail = new Node(e);
            head = tail;
        }else{
            tail.next = new Node(e);
            tail = tail.next;
        }
        size ++;
    }

    @Override
    public E dequeue(){
        if(isEmpty())
            throw new IllegalArgumentException("cannot dequeue from an empty queue");
        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if(head == null)
            tail = null;
        size --;

        return retNode.e;
    }

    @Override
    public E getFront(){
        if(isEmpty())
            throw new IllegalArgumentException("Queue is empty.");
        return head.e;
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Queue: front ");

        Node cur = head;
        while(cur != null){
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");
        return res.toString();
    }

    public static void main(String[] args) {
        LinkedListQueue<Integer> linkedListQueue = new LinkedListQueue<>();
        for(int i = 0;i < 10;i ++){
            linkedListQueue.enqueue(i);
            System.out.println(linkedListQueue);

            if(i % 3 == 2){
                linkedListQueue.dequeue();
                System.out.println(linkedListQueue);
            }
        }
//        Queue: front 0->NULL tail
//        Queue: front 0->1->NULL tail
//        Queue: front 0->1->2->NULL tail
//        Queue: front 1->2->NULL tail
//        Queue: front 1->2->3->NULL tail
//        Queue: front 1->2->3->4->NULL tail
//        Queue: front 1->2->3->4->5->NULL tail
//        Queue: front 2->3->4->5->NULL tail
//        Queue: front 2->3->4->5->6->NULL tail
//        Queue: front 2->3->4->5->6->7->NULL tail
//        Queue: front 2->3->4->5->6->7->8->NULL tail
//        Queue: front 3->4->5->6->7->8->NULL tail
//        Queue: front 3->4->5->6->7->8->9->NULL tail
    }
}

 11.链表队列和数组队列、循环队列进行比较

import java.util.Random;

public class Main {//测试使用q运行opCount个enqueue和dequeue操作所需要的时间,单位:秒
    private static double testQueue(Queue<Integer> q,int opCount){
        long startTime = System.nanoTime();
        Random random = new Random();
        for(int i = 0; i < opCount; i++)
            q.enqueue(random.nextInt(Integer.MAX_VALUE));
        for(int i = 0;i < opCount; i++)
            q.dequeue();
        long endTime = System.nanoTime();
        return (endTime - startTime) / 1000000000.0;
    }

    public static void main(String[] args) {
        int opCount = 100000;
        //数组队列
        ArrayQueue<Integer> arrayQueue = new ArrayQueue<>();
        double time1 = testQueue(arrayQueue,opCount);
        System.out.println("ArrayQueue, time:" + time1 + "s");       //ArrayQueue, time:4.859302024s

        //循环队列
        LoopQueue<Integer> loopQueue = new LoopQueue<>();
        double time2 = testQueue(loopQueue,opCount);
        System.out.println("LoopQueue, time:" + time2 + "s");        //LoopQueue, time:0.019878675s

        //链表队列
        LinkedListQueue<Integer> linkedListQueue = new LinkedListQueue<>();
        double time3 = testQueue(linkedListQueue,opCount);
        System.out.println("LinkedListQueue, time:" + time3 + "s");  //LinkedListQueue, time:0.015294216s
    }
}

 12.删除链表中的结点（https://leetcode-cn.com/problems/remove-linked-list-elements/description/）

删除链表中等于给定值 val 的所有节点。

示例:

输入: 1->2->6->3->4->5->6, val = 6
输出: 1->2->3->4->5

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        //删除头结点
        while(head != null && head.val == val){
            ListNode delNode = head;
            head = head.next;
            delNode.next = null;
        }

        if(head == null)
            return null;

        ListNode prev = head;
        while(prev.next != null){
            if(prev.next.val == val){
                ListNode delNode = prev.next;
                prev.next = delNode.next;
                delNode.next = null;
            }else{
                prev = prev.next;
            }
        }

        return head;
    }
}

进一步优化,增加虚拟头结点

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;

        ListNode prev = dummyHead;
        while(prev.next != null){
            if(prev.next.val == val){
                ListNode delNode = prev.next;
                prev.next = delNode.next;
                delNode.next = null;
            }else{
                prev = prev.next;
            }
        }

        return dummyHead.next;
    }
}

 调用实例:

public class Main {
    public static void main(String[] args) {
       int[] nums = {1, 2, 6, 3, 4, 5, 6};
       ListNode head = new ListNode(nums);
       System.out.println(head);  //1->2->6->3->4->5->6->NULL

       ListNode res = (new Solution()).removeElements(head , 6);
       System.out.println(res);   //1->2->3->4->5->NULL
    }
}