《算法》第四版答案（三）习题1.3

本章节为习题1.3.12-1.3.16的答案

• 1.3.12
  答案

在作者的Stack程序中添加如下方法：
    public static  Stack copy(Stack s)  
    {  
        Iterator it = s.iterator();  
        Stack result = new Stack();  
        Stack temp = new Stack();  
        while (it.hasNext()) {  
            temp.push(it.next());  
        }  
        it = temp.iterator();  
        while (it.hasNext()) {  
            result.push(it.next());  
        }  
        return result;  
    } 

测试代码：

public class ex1_3_12 {
        public static void main(String[] args) {  
            Stack s1 = new Stack();  
            s1.push("first");  
            s1.push("second");  
            s1.push("third");  
              
            Stack s2 = Stack.copy(s1); 
            while (!s2.isEmpty()) {  
                System.out.println(s2.pop());  
            }  
        }  
     
}

• 1.3.13
  答案

b c d

• 1.3.14
  答案

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

public class ResizingArrayQueue implements Iterable {
    private Item[] q;       // queue elements
    private int n;          // number of elements on queue
    private int first;      // index of first element of queue
    private int last;       // index of next available slot


    /**
     * Initializes an empty queue.
     */
    public ResizingArrayQueue() {
        q = (Item[]) new Object[2];
        n = 0;
        first = 0;
        last = 0;
    }

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

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

    // resize the underlying array
    private void resize(int capacity) {
        assert capacity >= n;
        Item[] temp = (Item[]) new Object[capacity];
        for (int i = 0; i < n; i++) {
            temp[i] = q[(first + i) % q.length];
        }
        q = temp;
        first = 0;
        last  = n;
    }

    /**
     * Adds the item to this queue.
     * @param item the item to add
     */
    public void enqueue(Item item) {
        // double size of array if necessary and recopy to front of array
        if (n == q.length) resize(2*q.length);   // double size of array if necessary
        q[last++] = item;                        // add item
        if (last == q.length) last = 0;          // wrap-around
        n++;
    }

    /**
     * Removes and returns the item on this queue that was least recently added.
     * @return the item on this queue that was least recently added
     * @throws java.util.NoSuchElementException if this queue is empty
     */
    public Item dequeue() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        Item item = q[first];
        q[first] = null;                            // to avoid loitering
        n--;
        first++;
        if (first == q.length) first = 0;           // wrap-around
        // shrink size of array if necessary
        if (n > 0 && n == q.length/4) resize(q.length/2); 
        return item;
    }

    /**
     * Returns the item least recently added to this queue.
     * @return the item least recently added to this queue
     * @throws java.util.NoSuchElementException if this queue is empty
     */
    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        return q[first];
    }


    /**
     * Returns an iterator that iterates over the items in this queue in FIFO order.
     * @return an iterator that iterates over the items in this queue in FIFO order
     */
    public Iterator iterator() {
        return new ArrayIterator();
    }

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

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = q[(i + first) % q.length];
            i++;
            return item;
        }
    }

   /**
     * Unit tests the {@code ResizingArrayQueue} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        ResizingArrayQueue queue = new ResizingArrayQueue();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (!item.equals("-")) queue.enqueue(item);
            else if (!queue.isEmpty()) StdOut.print(queue.dequeue() + " ");
        }
        StdOut.println("(" + queue.size() + " left on queue)");
    }

}

• 1.3.15
  答案

import java.util.Iterator;  
  
import edu.princeton.cs.algs4.StdIn;  
  
/** 
 * ClassName    : Queue 
 
 * Function     : TODO ADD FUNCTION. 
 
 * date         : Sep 28, 2016 10:21:54 AM 
 
 *  
 * @version  
 */  
public class Queue implements Iterable  
{  
    private Node first;  
    private Node last;  
    private int n;  
      
    private class Node  
    {  
        Item item;  
        Node next;  
    }  
      
    public boolean isEmpty()  
    {  
        return first == null;  
    }  
      
    public int size()  
    {  
        return n;  
    }  
      
    public void enqueue(Item item)  
    {  
        Node oldlast = last;  
        last = new Node();  
        last.item = item;  
        last.next = null;  
        if (isEmpty())  
        {  
            first = last;  
        }  
        else   
        {  
            oldlast.next = last;  
        }  
        n++;  
    }  
      
    public Item dequeue()  
    {  
        Item item = first.item;  
        first = first.next;  
        if (isEmpty())  
        {  
            last = null;  
        }  
        n--;  
        return item;  
    }  
      
    @Override  
    public Iterator iterator()  
    {  
        return new QueueIterator();  
    }  
      
    private class QueueIterator implements Iterator  
    {  
        private Node current = first;  
        @Override  
        public boolean hasNext()  
        {  
            return current != null;  
        }  
          
        @Override  
        public Item next()  
        {  
            Item item = current.item;  
            current = current.next;  
            return item;  
        }  
    }  
      
      
      
    public static void main(String[] args)  
    {  
        Queue q = new Queue();  
        while (!StdIn.isEmpty())  
        {  
            String item = StdIn.readString();  
            if (!item.equals("-"))  
            {  
                q.enqueue(item);  
            }  
            else if (!q.isEmpty())  
            {  
                System.out.print(q.dequeue() + " ");  
            }  
        }  
        System.out.println("(" + q.size() + " left on queue)");  
    }  
  
}  

测试:
public class E10315  
{  
    public static void main(String[] args)  
    {  
        int k = Integer.parseInt(args[0]);  
        Scanner scanner = new Scanner(System.in);  
        Queue q = new Queue();  
        while (scanner.hasNext()) {  
            q.enqueue(scanner.next());  
        }  
        scanner.close();  
          
        int size = q.size();  
        for (int i = 0; i < size - k; i++) {  
//            System.out.print(q.dequeue() + " ");  
            q.dequeue();  
        }  
        System.out.println(q.dequeue());  
    }  
}  

• 1.3.16
  答案

public class Date  
{  
    private final int month;  
    private final int day;  
    private final int year;  
      
    public Date(int m, int d, int y)  
    {  
        month = m;  
        day = d;  
        year = y;  
    }  
      
    public Date(String date)  
    {  
        String[] s = date.split("\\/");  
        if (s.length != 3) {  
            throw new IllegalArgumentException("Arguments illegal: " + date);  
        }  
        month = Integer.parseInt(s[0]);  
        day = Integer.parseInt(s[1]);  
        year = Integer.parseInt(s[2]);  
    }  
      
    public int month()  
    {  
        return month;  
    }  
      
    public int day()  
    {  
        return day;  
    }  
      
    public int year()  
    {  
        return year;  
    }  
      
    public String toString()  
    {  
        return month() + "/" + day() + "/" + year();  
    }  
      
    public boolean equals(Object x)  
    {  
        if (this == x) return true;  
        if (x == null) return false;  
        if (this.getClass() != x.getClass()) return false;  
        Date that = (Date)x;  
        if (this.day != that.day) return false;   
        if (this.month != that.month) return false;  
        if (this.year != that.year) return false;   
        return true;  
    }  
      
    public static Date[] readDates(String s)  
    {  
        String[] dates = s.split(" ");  
        int n = dates.length;  
        Queue q = new Queue();  
        for (int i = 0; i < n; i++) {  
            q.enqueue(new Date(dates[i]));  
        }  
          
        Date[] result = new Date[n];  
        for (int i = 0; i < n; i++) {  
            result[i] = q.dequeue();  
        }  
             
        return result;  
    }  
}  

测试：
    public class E10316  
    {  
        public static void main(String[] args)  
        {  
            String s = "11/30/2009 1/12/2012";  
            Date[] dates = Date.readDates(s);  
            for (int i = 0; i < dates.length; i++) {  
                System.out.println(dates[i]);  
            }  
        }  
    }