下压桟（LIFO）

　　

　　此代码并不复杂，但却是其他集合数据类型的实现的模版

import java.util.Iterator;

public class ResizingArrayStackStack<Item> implements Iterable<Item>{
    private Item[] a = (Item[]) new Object[1];
    private int N = 0;
    public boolean isEmpty(){
        return N==0;
    };

    public int size(){
        return N;
    };

    private 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 push(Item item){
        if(N == a.length)resize(2*a.length);
        a[N++] = item;
    }

    public Item pop(){
        Item item = a[--N];
        a[N] = null;
        if(N > 0 && N == a.length/4) resize(a.length/2);
        return item;
    }

    public Iterator<Item> iterator(){
        return new ReverseArrayIterator();
    }

    private class ReverseArrayIterator implements Iterator<Item>{
        private int i = N;
        
        public boolean hasNext(){ return i > 0;}
        public Item next()    { return a[--i];}
        public void remove()    {}
    }
}