4、链表
4.1 线性数据结构
底层依托静态数组,靠resize解决固定容量问题
- 动态数组
- 栈
- 队列
真正的动态数据结构
- 链表,不需要处理固定容量大的问题
4.2 链表的重要性
- 更深入的理解引用(或者指针)
- 更深入的理解递归
- 最简单的动态数据结构
- 辅助组成其他数据结构
4.3 链表
-
数据存储在节点Node中
class Node{ E e; Node next; }
- 丧失了随机访问的能力
- 数组和链表的对比
- 数组最好用于索引有语义的情况,最大的优点是支持快速查询
- 链表不适合具有索引语义的情况,最大的优点是动态
- 链表的时间复杂度分析:
- 添加addLast:O(n)
- 添加addFirst:O(1)
- 添加add(e,index):O(n/2) = O(n)
- 删除removeLast:O(n)
- 删除removeFirst:O(1)
- 删除remove(e,index):O(n/2) = O(n)
- 修改set(index):O(n)
- 查contains(e):O(n)
- 查find(e):O(n)
4.4 链表的实现
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();
}
}
//虚拟头结点
private Node dummyHead;
private int size;
public LinkedList() {
//初始化虚拟头结点
dummyHead = new Node(null,null);
size = 0;
}
public int getSize() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
//在链表中间添加元素
public void add(E e, int index) {
if(index <0 || index > size) {
throw new IllegalArgumentException("add failed");
}
Node prev = dummyHead;
for(int i = 0; i< index; i++)
prev = prev.next;
Node node = new Node(e);
node.next = prev.next;
prev.next = node;
size ++;
}
//在链表中删除元素
public E remove(int index) {
if(index <0 || index > size) {
throw new IllegalArgumentException("remove failed");
}
Node prev = dummyHead;
for(int i = 0;i<index; i++) {
prev = prev.next;
}
Node ret = prev.next;
prev.next = ret.next;
ret.next = null;
size--;
return ret.e;
}
public E removeFirst() {
return remove(0);
}
public E removeLast() {
return remove(size -1);
}
//链表头部添加元素
public void addFirst(E e) {
add(e,0);
}
//在链表末尾添加元素
public void addLast(E e) {
add(e,size);
}
//获取链表中的第index元素
public E get(int index) {
if(index <0 || index > size) {
throw new IllegalArgumentException("get failed");
}
//从第一个元素开始遍历
Node cur = dummyHead.next;
for(int i=0; i<index;i++) {
cur = cur.next;
}
return cur.e;
}
//获取链表的第一个元素
public E getFirst() {
return get(0);
}
public E getLast() {
return get(size - 1);
}
//修改链表的第index元素
public void set(E e, int index) {
if(index <0 || index > size) {
throw new IllegalArgumentException("set failed");
}
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();
}
}
4.5 链表测试
public class Main{
public static void main(String[] args) {
//测试链表
LinkedList<Integer> linkedList = new LinkedList<>();
for(int i = 0; i < 5; i++) {
linkedList.addFirst(i);
System.out.println(linkedList);
}
linkedList.add(666,2);
System.out.println(linkedList);
linkedList.remove(2);
System.out.println(linkedList);
linkedList.removeFirst();
System.out.println(linkedList);
linkedList.removeLast();
System.out.println(linkedList);
}
}
4.6 基于链表实现栈
public class LinkedListStack<E> implements Stack<E> {
private LinkedList<E> linkedList;
public LinkedListStack() {
linkedList = new LinkedList<>();
}
@Override
public int getSize() {
return linkedList.getSize();
}
@Override
public boolean isEmpty() {
return linkedList.isEmpty();
}
//入栈操作
@Override
public void push(E e) {
linkedList.addFirst(e);
}
//出栈操作
@Override
public void pop() {
return linkedList.removeFirst();
}
//得到栈顶元素
@Override
public E peek() {
return linkedList.getFirst();
}
@Override
public String toString() {
StringBuilder stringBuilder = new StringBuilder();
stringBuilder.append("top: ");
stringBuilder.append(linkedList);
stringBuilder.append(" :end");
return stringBuilder.toString();
}
}
4.7 测试链表栈
public class Main{
public static void main(String[] args) {
//测试链表栈
LinkedListStack<Integer> linkedListStack = new LinkedListStack<>();
for(int i = 0; i <5; i++) {
linkedListStack.push(i);
System.out.println(linkedListStack);
}
linkedListStack.pop();
System.out.println(linkedListStack);
}
}
4.8 测试对比链表栈与数组栈
两者消耗时间差不多
public class Main{
public static void main(String[] args) {
int opCount = 1000000;
double time1 = testStack(new ArrayStack<>(opCount),opCount);
double time2 = testStack(new LinkedListStack<>(),opCount);
System.out.println(time1);
System.out.println(time2);
}
public static double testStack(Stack<Integer> s, int opCount) {
long startTime = System.nanoTime();
Random random = new Random();
for(int i = 0; i < opCount; i++) {
s.push(random.nextInt(Integer.MAX_VALUE));
}
for(int i =0; i<opCount; i++) {
s.pop();
}
long endTime = System.nanoTime();
return (endTime - startTime) / 1000000000.0;
}
}
4.9 基于链表实现队列
链表实现队列的思想是在链表的首尾各添加一个指针,指向第一个节点和最后一个节点。这种设计思路原因是,链表取出第一个元素的复杂度为O(1),给链表添加一个尾指针指向尾元素后,添加元素复杂度为O(1),结合这两点正好可以分别利用到队列的出队与入队操作。其大致思想如下图所示:
实现方法:
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 0;
}
@Override
public boolean isEmpty() {
return false;
}
//链表尾端添加元素
@Override
public void enqueue(E e) {
//当链表没有元素时,要新建一个节点
if(tail == null) {
tail = new Node(e);
head = tail;
} else {
tail.next = new Node(e);
tail = tail.next;
}
size++;
}
//链表首端取出元素
@Override
public E dequeue() {
if(isEmpty()) {
throw new IllegalArgumentException("deque failed");
}
Node node = head;
head = head.next;
node.next = null;
//当链表中只有一个元素时,删除第一个节后,就没有节点了,此时tail需要指向null
if(head == null) {
tail = null;
}
size--;
return node.e;
}
@Override
public E getFront() {
if(isEmpty()) {
throw new IllegalArgumentException("deque failed");
}
return head.e;
}
@Override
public String toString() {
StringBuilder res = new StringBuilder();
res.append("Queue: front ");
Node node = head;
while(node != null) {
res.append(node+"->");
node = node.next;
}
res.append("Null tail");
return res.toString();
}
}
4.10 测试链表队列
public class Main{
public static void main(String[] args) {
//测试链表队列
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);
}
}
}
}