Algs4-1.3.33一个双向队列Deque-双向链表实现

1.3.33Deque。一个双向队列（或者称为deque)和栈或队列类似，但它同时支持在两端添加或删除元素。Deque能够存储一组元素并支持表1.3.9中的API:
表1.3.9泛型双向队列的API
public class Deque<Item> implements Iterable<Item>
    Deque()//创建空双向队列
    boolean isEmpty()//双向队列是否为空
    int size()//双向队列中的元素数量
    void pushLeft(Item item)//向左端添加一个新元素
    void pushRight(Item item)//向右端添加一个新元素
    Item popLeft() 从左端删除一个元素
    Item popRight()从右端删除一个元素
编写一个使用双向链表实现这份API的Deque类。
答:

import java.util.Iterator;
public class Deque<Item> implements Iterable<Item>
{
   private class Node
    {
        Item item;
        Node prev;
        Node next;
    }
   
   private int N;
   private Node left;
   private Node right;
  
    public Deque()
    {
        left=null;
        right=null;
        N=0;
    }
   
    public boolean isEmpty()
    {return N==0;}
   
    public int size()
    {return N;}
   
    public void pushLeft(Item item)
    {
        Node oldLeft=left;
        left=new Node();
        left.item=item;
        left.prev=null;
        left.next=oldLeft;
      
        if(isEmpty())
            right=left;
        else
             oldLeft.prev=left;
        N++;
    }
   
    public void pushRight(Item item)
    {
        Node oldRight=right;
        right=new Node();
        right.item=item;
        right.prev=oldRight;
        right.next=null;
        if(isEmpty())
            left=right;
        else
            oldRight.next=right;
           
        N++;
    }
   
    public Item popLeft()
    {
        Item item;
       if(size()==0)
       {
           item=null;
       }
       else if(size()==1)
       {
           item=left.item;
           left=null;
           right=null;
           N--;
       }
       else
       {
           Node oldLeft=left;
           left=left.next;
           left.prev=null;
           oldLeft.next=null;
           item=oldLeft.item;
           N--;
       }
        return item;
    }
   
    public Item popRight()
    {
        Item item;
        if(size()==0)
        {
            item=null;
        }
        else if(size()==1)
        {
            item=right.item;
            left=null;
            right=null;
            N--;
        }
        else
        {
            Node oldRight=right;
            right=right.prev;
            oldRight.prev=null;
            right.next=null;
            item=oldRight.item;
            N--;
         }
        return item;
    }
 
    public Iterator<Item> iterator()  {return new ListIterator();}
   
    private class ListIterator implements Iterator<Item>
    {
        private Node current=left;
        public boolean hasNext(){return current!=null;}
        public void remove(){}
        public Item next()
        {
            Item item=current.item;
            current=current.next;
            return item;
        }//end next
      }//end class ListIterator
   
    public static void main(String[] args)
    {
        Deque<String> q=new Deque<String>();
        In in1=new In(args[0]);
        In in2=new In(args[0]);
        In in3=new In(args[0]);
        In in4=new In(args[0]);
        //pushLeft and popLeft
   
        StdOut.printf(" ---pushLeft and popLeft--- ");
        while(!in1.isEmpty())
        {
            String item=in1.readString();
            q.pushLeft(item);
        }
         while(!q.isEmpty())
        {
        StdOut.print(q.popLeft()+" ");
        }
   
        //pushLeft and popRight
   
        StdOut.printf(" ---pushLeft and popRight--- ");
        while(!in2.isEmpty())
        {
            String item=in2.readString();
            q.pushLeft(item);
        }

        while(!q.isEmpty())
        {
        StdOut.print(q.popRight()+" ");
        }
    
        //pushRight and popLeft
        StdOut.printf(" ---pushRight and popLeft--- ");
        while(!in3.isEmpty())
        {
            String item=in3.readString();
            q.pushRight(item);
        }

        while(!q.isEmpty())
        {
        StdOut.print(q.popLeft()+" ");
        }
     

        //pushRight and popRight
        StdOut.printf(" ---pushRight and popRight--- ");
        while(!in4.isEmpty())
        {
            String item=in4.readString();
            q.pushRight(item);
        }

        while(!q.isEmpty())
        {
        StdOut.print(q.popRight()+" ");
        }
 
    }
}