[数据结构与算法]数据结构day07

队列

• 队列也是一种线性结构
• 相比数组队列对应的操作是数组的子集
• 只能从一端(队尾)添加元素, 只能从另一端(队首)取出元素
  比如去银行办业务
  队列是一种先进先出的数据结构,(FIFO)
  接口

package com.qcx.algo.day07.implqueue;

public interface Queue<E> {
    int getSize();
    boolean isEmpty();
    // 入队
    void enqueue(E e);
    // 出队
    E dequeue();
    // 获取队首是元素
    E getFront();
}

实现类

package com.qcx.algo.day07.implqueue;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;

public class ArrayQueue<E> implements Queue<E>{

    private Array<E> array;

    public ArrayQueue(int capacity){
        array = new Array<>(capacity);
    }
    public ArrayQueue(){
       array = new Array<>();
    }

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

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

    @Override
    public void enqueue(E e) {
        array.addLast(e);
    }

    @Override
    public E dequeue() {
        return array.removeFirst();
    }

    @Override
    public E getFront() {
        return array.getFirst();
    }
    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: ");
        res.append("front [");
        for (int i = 0; i < array.getSize(); i++) {
            res.append(array.get(i));
            if(i != array.getSize() - 1)
                res.append(", ");
        }
        res.append("]  tail");
        return res.toString();
    }

    public static void main(String[] args) {
        Queue<Integer> queue = new ArrayQueue<>();
//        List<Object> list = new ArrayList<>();
//        List<Object> list2 = new LinkedList<>();
        for (int i = 0; i < 10; i++) {
            queue.enqueue(i);
            System.out.println(queue);
            if (i%3==2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}

时间复杂度分析

int getSize();   o(1)
boolean isEmpty(); o(1)
// 入队
void enqueue(E e); o(1)
// 出队
E dequeue();  o(n)
// 获取队首是元素
E getFront(); o(1)

循环队列

原来出队是需要都往前移动一下, 循环队列只需要维护一下 front和tail两个值就行了
我们先指定一个标准
front == tail 队列为空
front == tail + 1 队列为满 (此时浪费了一个空间)

( tail + 1)%capacity == front 队列为满
(9 + 1) %10 == 0
(10+ 1) %10 == 1

package com.qcx.algo.day07.implqueue;

public class LoopQueue<E> implements Queue<E>{
    private E[] data;
    private int front, tail;
    private int size;
    public LoopQueue(int capacity){
        data =  (E[]) new Object[capacity + 1];
        front = 0;
        tail = 0;
        size = 0;
    }

    public LoopQueue(){
        this(10);
    }

    public int getCapacity(){
        return data.length - 1;
    }

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

    @Override
    public boolean isEmpty() {
        return front==tail;
    }

    @Override
    public void enqueue(E e) {
        // 先判断是否满了
        if ((tail + 1)%data.length == front)
            resize(getCapacity()*2);
        data[tail] = e;
        tail = (tail + 1)%data.length;
        size++;

    }
   private void resize(int newCapacity){
       E[]  newData =  (E[]) new Object[newCapacity + 1];
       for (int i = 0; i < size; i++) {
           newData[i] = data[(i + front)% data.length];
       }
       data = newData;
       front = 0;
       tail = size;
    }
    @Override
    public E dequeue() {
        if (isEmpty())
            throw new IllegalArgumentException("Cannot dequeue from an empty LoopQueue");
        // 保存一下队首的值
        E ret = data[front];
        data[front] = null;
        front = (front+1)% data.length;
        size--;
        if (size == getCapacity() / 4 && getCapacity()/2 !=0){
            resize(getCapacity()/2);
        }
        return ret;

    }

    @Override
    public E getFront() {
        if (isEmpty())
            throw new IllegalArgumentException("Cannot dequeue from an empty LoopQueue");
        return data[front];
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: ");
        res.append("front [");
        for (int i = front; i !=tail ; i = (i + 1)% data.length) {
            res.append(data[i]);
            if ((i + 1)% data.length != tail)
                res.append(", ");

        }
        res.append("]  tail");
        return res.toString();
    }
    public static void main(String[] args) {
        LoopQueue<Integer> loopQueue = new LoopQueue<>();
        for (int i = 0; i < 10; i++) {
            loopQueue.enqueue(i);
            System.out.println(loopQueue);
            if (i%3==2){
                loopQueue.dequeue();
                System.out.println(loopQueue);
            }
        }
    }
}

测试

package com.qcx.algo.day07.implqueue;

import java.util.Random;

public class Main {
    // 返回代码巡行的时间  单位是s
    public 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)/100000000.0;
    }

    public static void main(String[] args) {
        int opCount = 100000;
        Queue<Integer> arrayQueue = new ArrayQueue<>();
        double time1 = testQueue(arrayQueue, opCount);
        System.out.println("arrayQueue time = " + time1);
        Queue<Integer> loopQueue = new LoopQueue<>();
        double time2 = testQueue(loopQueue, opCount);

        System.out.println("loopQueue time = " + time2);

    }
}