何为队列
队列是一种特殊的线性表只可以在头尾两端进操作
队尾(rear):只能从队尾添加元素,叫入队
队头(front):只可以从队头取出或则删除元素,叫出队
先进先出原则:First in First out FIFO
就好比我们生活中在食堂里排队买饭,先来的人先买饭,后来的人排在队尾
队列的接口设计
int size(); //元素的数量
boolean(); //是否为空
void clear();//清空元素
void enQueue(E element);//入队
E deQueue(); //出队
E front();//获取队列的头元素
**
用链表实现队列
LinkedList源码
package list;
import list.AbstractList;
public class LinkedList<E> extends AbstractList<E> {
private Node<E> first;
private Node<E> last;
private static class Node<E> {
E element;
Node<E> prev;
Node<E> next;
public Node(Node<E> prev, E element, Node<E> next) {
this.prev = prev;
this.element = element;
this.next = next;
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
if (prev != null) {
sb.append(prev.element);
} else {
sb.append("null");
}
sb.append("_").append(element).append("_");
if (next != null) {
sb.append(next.element);
} else {
sb.append("null");
}
return sb.toString();
}
}
@Override
public void clear() {
size = 0;
first = null;
last = null;
}
@Override
public E get(int index) {
return node(index).element;
}
@Override
public E set(int index, E element) {
Node<E> node = node(index);
E old = node.element;
node.element = element;
return old;
}
@Override
public void add(int index, E element) {
rangeCheckForAdd(index);
// size == 0
// index == 0
if (index == size) { // 往最后面添加元素
Node<E> oldLast = last;
last = new Node<E>(oldLast, element, null);
if (oldLast == null) { // 这是链表添加的第一个元素
first = last;
} else {
oldLast.next = last;
}
} else {
Node<E> next = node(index);
Node<E> prev = next.prev;
Node<E> node = new Node<E>(prev, element, next);
next.prev = node;
if (prev == null) { // index == 0
first = node;
} else {
prev.next = node;
}
}
size++;
}
@Override
public E remove(int index) {
rangeCheck(index);
Node<E> node = node(index);
Node<E> prev = node.prev;
Node<E> next = node.next;
if (prev == null) { // index == 0
first = next;
} else {
prev.next = next;
}
if (next == null) { // index == size - 1
last = prev;
} else {
next.prev = prev;
}
size--;
return node.element;
}
@Override
public int indexOf(E element) {
if (element == null) {
Node<E> node = first;
for (int i = 0; i < size; i++) {
if (node.element == null) return i;
node = node.next;
}
} else {
Node<E> node = first;
for (int i = 0; i < size; i++) {
if (element.equals(node.element)) return i;
node = node.next;
}
}
return ELEMENT_NOT_FOUND;
}
/**
* 获取index位置对应的节点对象
* @param index
* @return
*/
private Node<E> node(int index) {
rangeCheck(index);
if (index < (size >> 1)) {
Node<E> node = first;
for (int i = 0; i < index; i++) {
node = node.next;
}
return node;
} else {
Node<E> node = last;
for (int i = size - 1; i > index; i--) {
node = node.prev;
}
return node;
}
}
@Override
public String toString() {
StringBuilder string = new StringBuilder();
string.append("size=").append(size).append(", [");
Node<E> node = first;
for (int i = 0; i < size; i++) {
if (i != 0) {
string.append(", ");
}
string.append(node);
node = node.next;
}
string.append("]");
return string.toString();
}
}
Queue实现
import com.sun.xml.internal.fastinfoset.util.ValueArrayResourceException;
import list.LinkedList;
import list.List;
public class Queue<E>{
private List<E> list=new LinkedList<E>();
//元素的数量
public int size (){
return list.size();
}
//是否为空
public boolean isEmpty(){
return list.isEmpty();
}
//清空元素
public void clear(){
list.clear();
}
//入队
public void enQueue(E element){
list.add(element); ;
}
//出队
public E deQueue(){
return list.remove(0);
}
//获取队列的头元素
public E front(){
return list.get(0);
}
}
双端队列实现
何为双端队列
可以在队头和队尾添加删除元素的队列
int size(); //元素的数量
boolean(); //是否为空
void clear();//清空元素
void enQueue(E element);//入队
E deQueue(); //出队
E front();//获取队列的头元素
比原有的以上方法多了以下方法
void enQueueRear(E element) //从队尾入队
E deQueeuRear() //从队尾出队
void enQueueFront() ///从队头入队
void rear(); //获取队尾元素
Dequeu源码实现
其中Linkedlist源码与上面的相同
import list.LinkedList;
import list.List;
public class Deque<E> {
private List<E> list=new LinkedList<E>();
//元素的数量
public int size (){
return list.size();
}
//是否为空
public boolean isEmpty(){
return list.isEmpty();
}
//清空元素
public void clear(){
list.clear();
}
//入队
public void enQueue(E element){
list.add(element); ;
}
//出队
public E deQueue(){
return list.remove(0);
}
//获取队列的头元素
public E front(){
return list.get(0);
}
/*从队尾入队*/
public void enQueueRear(E element) {
list.add(element);
}
/*从队尾出队*/
public E deQueueRear() {
return list.remove(list.size() - 1);
}
/*获取队尾元素*/
public E rear() {
return list.get(list.size() - 1);
}
/*从队头入队*/
public void enQueueFront(E element){
list.add(0,element);
}
}
循环队列
大致介绍
队列底层可以用数组实现并且优化后的队列也叫循环队列,即可以充分的利用空间,不浪费资源
删除一个元素
添加两个元素个元素
此时队列的后面已经 没有空间,但是前面还有一个空间
添加一个元素
这就是循环队列的大致实现即充分的保证不浪费数组的空间
源码实现
import com.sun.org.apache.bcel.internal.generic.NEW;
import org.omg.IOP.ENCODING_CDR_ENCAPS;
import javax.print.DocFlavor;
import java.security.interfaces.ECKey;
import java.util.EventListener;
public class CircleQueue<E> {
private int front;
private int size;
/*存储元素的数组*/
private E[] elements;
/*默认数组大小*/
private static final int DEFAULT_CAPACITY = 10;
/*构造方法*/
public CircleQueue() {
E[] es = (E[]) new Object[DEFAULT_CAPACITY];
}
//返回队列的大小
public int size() {
return size;
}
/*判断队列是否为空*/
public boolean isEmpty() {
return size == 0;
}
/*清除队列*/
public void clear() {
/*将数组元素设置为null*/
for (int i = 0; i < size; i++) {
elements[index(i)]=null;
}
/*头元素与size都置为0*/
front=0;
size=0;
}
/*元素入队*/
public void enQueue(E element) {
ensureCapacity(size+1);
elements[index(size)]=element;
size++;
}
/*元素出队*/
public E deQueue() {
/*保存队首元素*/
E element = elements[front];
/*队首元素置为null*/
elements[front]=null;
/*获取新的队首元素所在的地方*/
front=(index(1));
size--;
return element;
}
/*获取头元素*/
public E front() {
return elements[front];
}
@Override
public String toString() {
StringBuilder string = new StringBuilder();
string.append("capcacity=").append(elements.length)
.append(" size=").append(size)
.append(" front=").append(front)
.append(", [");
for (int i = 0; i < elements.length; i++) {
if (i != 0) {
string.append(", ");
}
string.append(elements[i]);
}
string.append("]");
return string.toString();
}
/*索引映射封装*/
private int index(int index) {
index+=front;
return index-(index>= elements.length?elements.length:0);
}
/**
* 保证要有capacity的容量
* @param capacity
*/
private void ensureCapacity(int capacity) {
int oldCapacity = elements.length;
if (oldCapacity>=capacity){
int newCapacity=oldCapacity+(oldCapacity>>1);
E[] newElements = (E[]) new Object[newCapacity];
for (int i = 0; i < size; i++) {
newElements[i]=elements[index(size)];
}
elements=newElements;
front=0;
}
}
}
重点方法
索引映射封装
就是一个简单的取模运算,不过我这里用了加减的方法来实现,学过计算机系统基础的同学都知道,这样子效率更高如果当前的索引值+front的值大于了当前数组的长度,说明该值在数组的前面
private int index(int index) {
index+=front;
return index-(index>= elements.length?elements.length:0);
}
数组扩容
private void ensureCapacity(int capacity) {
int oldCapacity = elements.length;
if (oldCapacity >= capacity) return;
// 新容量为旧容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
E[] newElements = (E[]) new Object[newCapacity];
for (int i = 0; i < size; i++) {
newElements[i] = elements[index(i)];
}
elements = newElements;
// 重置front
front = 0;
}
双端循环队列
package com.mj.circle;
@SuppressWarnings("unchecked")
public class CircleDeque<E> {
private int front;
private int size;
private E[] elements;
private static final int DEFAULT_CAPACITY = 10;
public CircleDeque() {
elements = (E[]) new Object[DEFAULT_CAPACITY];
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
public void clear() {
for (int i = 0; i < size; i++) {
elements[index(i)] = null;
}
front = 0;
size = 0;
}
/**
* 从尾部入队
* @param element
*/
public void enQueueRear(E element) {
ensureCapacity(size + 1);
elements[index(size)] = element;
size++;
}
/**
* 从头部出队
* @param element
*/
public E deQueueFront() {
E frontElement = elements[front];
elements[front] = null;
front = index(1);
size--;
return frontElement;
}
/**
* 从头部入队
* @param element
*/
public void enQueueFront(E element) {
ensureCapacity(size + 1);
front = index(-1);
elements[front] = element;
size++;
}
/**
* 从尾部出队
* @param element
*/
public E deQueueRear() {
int rearIndex = index(size - 1);
E rear = elements[rearIndex];
elements[rearIndex] = null;
size--;
return rear;
}
public E front() {
return elements[front];
}
public E rear() {
return elements[index(size - 1)];
}
@Override
public String toString() {
StringBuilder string = new StringBuilder();
string.append("capcacity=").append(elements.length)
.append(" size=").append(size)
.append(" front=").append(front)
.append(", [");
for (int i = 0; i < elements.length; i++) {
if (i != 0) {
string.append(", ");
}
string.append(elements[i]);
}
string.append("]");
return string.toString();
}
private int index(int index) {
index += front;
if (index < 0) {
return index + elements.length;
}
return index - (index >= elements.length ? elements.length : 0);
}
/**
* 保证要有capacity的容量
* @param capacity
*/
private void ensureCapacity(int capacity) {
int oldCapacity = elements.length;
if (oldCapacity >= capacity) return;
// 新容量为旧容量的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);
E[] newElements = (E[]) new Object[newCapacity];
for (int i = 0; i < size; i++) {
newElements[i] = elements[index(i)];
}
elements = newElements;
// 重置front
front = 0;
}
}