Algs4-1.3.33一个双向队列Deque的可变长环形数组实现

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
编写一个使用动态数组调整实现这份API的ResizeArrayDeque类。
答：环形时无论left<right 还是right>left，pushLeft 均为left--,pushRight均为right++,popLeft均为left++,popRight均为right--,当left或right值为-1时值置为 array.length-1,当left或right值为array.length时值置为0。


import java.util.Iterator;
public class ResizeArrayDeque<Item> implements Iterable<Item>
{
   private Item[] a=(Item[]) new Object[1];
   private int N;
   private int left;
   private int right;
  
    public ResizeArrayDeque()
    {
        left=0;
        right=0;
        N=0;
    }
   
    public boolean isEmpty()
    {return N==0;}
   
    public int size()
    {return N;}
   
    public void pushLeft(Item item)
    {
        if(N==a.length) resize(2*N);
        left--;
        if(left==-1) left=a.length-1;
        a[left]=item;
        if(isEmpty()) right=left;
        N++;
    }
   
    public void pushRight(Item item)
    {
       if(N==a.length) resize(2*N);
        right++;
        if(right==a.length) right=0;
        a[right]=item;
        if(isEmpty()) left=right;
        N++;
    }
   
    public Item popLeft()
    {
        Item item;
       if(size()==0)
       {
           item=null;
       }
       else if(size()==1)
       {
           item=a[left];
           N--;
       }
       else
       {
           item=a[left];
           left++;
           if(left==a.length) left=0;
           N--;
           if(N==a.length/4) resize(2*N);
          
       }
        return item;
    }
   
    public Item popRight()
    {
        Item item;
        if(size()==0)
        {
            item=null;
        }
        else if(size()==1)
        {
            item=a[right];
            N--;
        }
        else
        {
            item=a[right];
            right--;
            if(right==-1) right=a.length-1;
            N--;
            if(N==a.length/4) resize(2*N);
         }
        return item;
    }
   
    private void resize(int max)
    {
        Item[] temp=(Item[]) new Object[max];
        int j=left;
        for(int i=0;i<N;i++)
        {
            temp[i]=a[j];
            j++;
            if(j==a.length) j=0;
        }
        a=temp;
        left=0;
        right=N-1;
    }
 
    public Iterator<Item> iterator()  {return new ListIterator();}
   
    private class ListIterator implements Iterator<Item>
    {
        private int current=left;
        public boolean hasNext(){return current!=right+1;}
        public void remove(){}
        public Item next()
        {
            Item item=a[current];
            current++;
            if(current==a.length) current=0;
            return item;
        }//end next
      }//end class ListIterator
   
    public static void main(String[] args)
    {
        ResizeArrayDeque<String> q=new ResizeArrayDeque<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
       
        while(!in1.isEmpty())
        {
            String item=in1.readString();
            q.pushLeft(item);
        }
        StdOut.printf(" ---pushLeft and for each Deque--- ");
        for(String i:q)
            StdOut.print(i+" ");
       
        StdOut.printf(" ---pushLeft and popLeft--- ");
        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()+" ");
        }

    }
}