Deque.java

import java.util.NoSuchElementException;

import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;

import java.util.Iterator;

public class Deque<Item> implements Iterable<Item> {
    private Node first;
    private Node last;
    private int size;

    // inner node class
    private class Node {
        private Item item;
        private Node next;
        private Node prev;
    }

    // intializes an empty deque
    public Deque() {
        first = null;
        last = null;
        size = 0;
        assert check();
    }

    // checks if deque contains any items
    public boolean isEmpty() {
        return first == null;
    }

    // returns number of items in deque
    public int size() {
        return size;
    }

    // adds item to front of the deck
    public void addFirst(Item item) {
        if (isEmpty()) {
            first = new Node();
            last = first;
            first.item = item;
        } else {
            Node oldFirst = first;
            first = new Node();
            first.item = item;
            first.next = oldFirst;
            oldFirst.prev = first;
        }
        size++;
        assert check();
    }

    public void addLast(Item item) {
        if (isEmpty()) {
            last = new Node();
            first = last;
            first.item = item;
        } else {
            Node oldLast = last;
            last = new Node();
            last.item = item;
            last.prev = oldLast;
            oldLast.next = last;
        }
        size++;
        assert check();
    }

    public Item removeFirst() {
        if (isEmpty())
            throw new NoSuchElementException("Deque is empty");
        Item item = first.item;
        if (size == 1) {
            first = null;
            last = null;
        } else {
            first = first.next;
            first.prev.next = null;
            first.prev = null;
        }
        size--;
        return item;
    }

    public Item removeLast() {
        if (isEmpty())
            throw new NoSuchElementException("Deque is empty");
        Item item = last.item;
        if (size == 1) {
            first = null;
            last = null;
        } else {
            last = last.prev;
            last.next.prev = null;
            last.next = null;
        }
        size--;
        return item;
    }

    private class DequeIterator implements Iterator<Item> {
        private Node 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;
        }
    }

    // check internal invariants
    private boolean check() {

        if (size < 0) {
            return false;
        }
        if (size == 0) {
            if (first != null)
                return false;
        } else if (size == 1) {
            if (first == null)
                return false;
            if (first.next != null)
                return false;
        } else {
            if (first == null)
                return false;
            if (first.next == null)
                return false;
        }

        // check internal consistency of instance variable N
        int numberOfNodes = 0;
        for (Node x = first; x != null && numberOfNodes <= size; x = x.next) {
            numberOfNodes++;
        }
        if (numberOfNodes != size)
            return false;

        return true;
    }

    // unit testing
    public static void main(String[] args) {
        Deque<String> d = new Deque<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (item.equals("tb"))
                d.addLast(item);
            else if (item.equals("tf"))
                d.addFirst(item);
            else if (item.equals("rb"))
                StdOut.println(d.removeLast());
            else if (item.equals("rf"))
                StdOut.println(d.removeFirst());
        }
        StdOut.print(" size: " + d.size());
    }

    @Override
    public Iterator<Item> iterator() {
        return new DequeIterator();
    }

}