十一 队列 Queue

队列：  一种先进先出的数据结构  FIFO

数组队列的实现：

队列的实现：

package com.lt.datastructure.Queue;

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

    private Array1<E> array;
    public ArrayQueue(int capacity){
        array = new Array1<>(capacity);
    }

    public ArrayQueue(){
        array = new Array1<>();
    }

    @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
    //查看队首
    public E getFront(E e) {
        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) {
        ArrayQueue<Integer> queue = new ArrayQueue<>();
        for(int i =0; i<10; i++){
            queue.enqueue(i);
            System.out.println(queue);
            if(i%3==2){
                queue.dequeue();
                System.out.println(queue);
            }
        }

    }
}

测试：

复杂度;

dequeue:出列操作时，移除队首，其余元素向前移一个位置  O(n)

循环队列：犹如钟表

• front指向队首，tail指向队尾
• 出队时，移动front的指向。
• font==tail时，队列为空
• （tail+1）%Capacity=front,队列满，会有一个元素浪费

循环队列复杂度分析：

循环队列的实现：

package com.lt.datastructure.Queue;

import com.lt.datastructure.Array.Array;
/*
 * 循环队列的实现
 */
public class LoopQueue<E> implements Queue<E> {

    private int front,tail;
    private int size;
    E[] data;

    public LoopQueue(int capacity){
        data = (E[])new Object[capacity+1];
        front = 0;
        tail  = 0;
        size = 0;
    }
    public LoopQueue(){
        this(10);
    }

    //由于空出一个空间，所以容量-1
    public int getCapacity() {
        return data.length-1;
    }

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

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

    @Override
    public void enqueue(E e) {
        //队列满，扩容
        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() {
        //查看的队列是否为空
        if(isEmpty()){
            throw new IllegalAccessError("cannot dequeue from an empty queue.");
        }
        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(E e) {
        //查看的队列是否为空
        if(isEmpty()){
            throw new IllegalAccessError("cannot getFront from an empty queue.");
        }
        return data[front];
    }

     @Override//覆盖父类方法
     public String toString(){
         StringBuilder res = new StringBuilder();
         res.append(String.format("LoopQueue:size = %d , capacity = %d\n",size,getCapacity()));
         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("]");
         return res.toString();
     }

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

                if(i%3==2){
                    queue.dequeue();
                    System.out.println(queue);
                }
            }
        }
}   

测试用例：

package com.lt.datastructure.Queue;

import java.util.Random;

    public class Main {
         /*
         * 测试ArrayQueue和LoopQueue
         */
        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;
      ArrayQueue<Integer> arrayQueue = new ArrayQueue<>();
      System.out.println("ArrayQueue:"+testQueue(arrayQueue,opCount));

      LoopQueue<Integer> loopQueue = new LoopQueue<>();
      System.out.println("LoopQueue:"+testQueue(loopQueue, opCount));

   }
}

测试结果：