一、定义

背包是一种不支持从中删除元素的集合数据类型——它的目的就是帮助用例收集元素并迭代遍历所有收集到的元素。背包的迭代顺序不确定且与用例无关。

1-1 背包示意图

二、API

2-1 背包的API定义

三、实现

3.1 数组方式实现

可动态调整大小的背包实现（数组方式）：

• 当包容量满时，采用遍历数组的方式进行扩容，默认调整为原容量的2倍。
• 当包容量变成1/4时，默认调整为原容量的1/2。

public class ResizingArrayBag<Item> implements Iterable<Item> {
    private Item[] a;         // array of items
    private int n;            // number of elements on bag

    /**
     * Initializes an empty bag.
     */
    public ResizingArrayBag() {
        a = (Item[]) new Object[2];
        n = 0;
    }

    /**
     * Is this bag empty?
     * @return true if this bag is empty; false otherwise
     */
    public boolean isEmpty() {
        return n == 0;
    }

    /**
     * Returns the number of items in this bag.
     * @return the number of items in this bag
     */
    public int size() {
        return n;
    }

    // resize the underlying array holding the elements
    private void resize(int capacity) {
        assert capacity >= n;
        Item[] temp = (Item[]) new Object[capacity];
        for (int i = 0; i < n; i++)
            temp[i] = a[i];
        a = temp;
    }

    /**
     * Adds the item to this bag.
     * @param item the item to add to this bag
     */
    public void add(Item item) {
        if (n == a.length) resize(2*a.length);    // double size of array if necessary
        a[n++] = item;                            // add item
    }


    /**
     * Returns an iterator that iterates over the items in the bag in arbitrary order.
     * @return an iterator that iterates over the items in the bag in arbitrary order
     */
    public Iterator<Item> iterator() {
        return new ArrayIterator();
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ArrayIterator implements Iterator<Item> {
        private int i = 0;
        public boolean hasNext()  { return i < n;                               }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            return a[i++];
        }
    }
}

3.2 链表方式实现

背包的链表实现方式：

• 添加元素相当于“头插法”插入元素。

public class Bag<Item> implements Iterable<Item> {
    private Node<Item> first;    // beginning of bag
    private int n;               // number of elements in bag

    // helper linked list class
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    /**
     * Initializes an empty bag.
     */
    public Bag() {
        first = null;
        n = 0;
    }

    /**
     * Returns true if this bag is empty.
     *
     * @return {@code true} if this bag is empty;
     *         {@code false} otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in this bag.
     *
     * @return the number of items in this bag
     */
    public int size() {
        return n;
    }

    /**
     * Adds the item to this bag.
     *
     * @param  item the item to add to this bag
     */
    public void add(Item item) {
        Node<Item> oldfirst = first;
        first = new Node<Item>();
        first.item = item;
        first.next = oldfirst;
        n++;
    }


    /**
     * Returns an iterator that iterates over the items in this bag in arbitrary order.
     *
     * @return an iterator that iterates over the items in this bag in arbitrary order
     */
    public Iterator<Item> iterator()  {
        return new ListIterator<Item>(first);  
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }
}