队列基础

用可变数组长度实现队列，用链实现队列

import java.util.Iterator;


public class ResizingArrayQueue<Item> implements Iterable<Item> {
    
    private Item[] a = (Item[])new Object[1];
    private int N = 0;
    
    public int size() {
        return N;
    }
    
    public boolean isEmpty() {
        return N == 0;
    }
    
    public void resize(int max) {
        Item[] temp = (Item[]) new Object[max];
        for(int i=0; i<N; i++) {
            temp[i] = a[i];
        }
        a = temp;
    }
    
    public void enqueue(Item item) {
        if(N == a.length) {
            resize(2*a.length);
        }
        a[N++] = item;
    }
    
    public Item dequeue() {
        //拷贝所有的老数组元素到新的数组，并命名为oldItems
        Item[] oldItems = (Item[]) new Object[N];
        for(int i=0; i<N; i++) {
            oldItems[i] = a[i];
        }
        
        //重新设置Queue的大小
        a[--N] = null;
        if(N>0 && N == a.length/4 ) {
            resize(a.length/2);
        }
        
        //获取新数组
        for(int i=0; i<N-1; i++) {
            a[i] =oldItems[i+1];
        }
        
        return oldItems[0];
    }
    
    @Override
    public Iterator<Item> iterator() {
        return new ArrayIterator();
    }
    
    private class ArrayIterator implements Iterator<Item>{
        
        private int i = N;

        @Override
        public boolean hasNext() {
            
            return i > 0;
        }

        @Override
        public Item next() {
            Item[] oldItems = (Item[]) new Object[N];
            for(int i=0; i<N; i++) {
                oldItems[i] = a[i];
            }
            a[--i] = null;
            for(int i=0; i<N-1; i++) {
                a[i] =oldItems[i+1];
            }
            return oldItems[0];
        }
        
    }

}

import java.util.Iterator;

public class Queue<Item> implements Iterable<Item> {
    
    private Node first;  //指向最早添加的结点的链接
    private Node last;   //指向最近添加的结点的链接
    private int N;       //队列中的元素数量
    
    private class Node{
        //定义了结点的嵌套类
        Item item;
        Node next;
    }
    
    public boolean isEmpty() {
        return N == 0;//或：first == null;
    }
    
    public int size() {
        return N;
    }
    
    public void enqueue(Item item) {
        //向表尾添加元素
        Node oldLast = last;
        last = new Node();
        last.item = item;
        last.next = null;
        if(isEmpty()) {
            first = last;
        }else {
            oldLast.next = last;
        }
        N++;
    }
    
    public Item dequeue() {
        //从表头删除元素
        Item item = first.item;
        first = first.next;
        if(isEmpty()) {
            last = null;
        }
        N--;
        return item;
    }
    
    @Override
    public Iterator<Item> iterator() {
        return new ListIterator();
    }
    
    private class ListIterator implements Iterator<Item>{
        
        private Node current = first;
        
        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public Item next() {
            Item item = current.item;
            current = current.next;
            if(isEmpty()) {
                last = null;
            }
            return item;
        }
        
    }

}