Bag Stack Queue

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Just the code:
Bag

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Package edu.princeton.cs.algs4;

import java.util.Iterator;
import java.util.NoSuchElementException;

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 <tt>true</tt> if this bag is empty;
     *         <tt>false</tt> 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;
        }
    }

    /**
     * Unit tests the <tt>Bag</tt> data type.
     */
    public static void main(String[] args) {
        Bag<String> bag = new Bag<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            bag.add(item);
        }

        StdOut.println("size of bag = " + bag.size());
        for (String s : bag) {
            StdOut.println(s);
        }
    }
}

stack and queue are similar.

array implementation
resizing array:
Q: how to grow array？
A: if array is full, create a new array of twice the size, and copy items
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