TelephoneDirectory

/*

Problem


You will have to design a telephone directory where the following operations would be supported.
1. GET: It will provide a number which is not assigned to anyone
2. CHECK: Given a number it will tell whether it is assigned to anyone or not
3. RELEASE: It will recycle or release a number

*/

public class TelephoneDirectory
{
 public static void main(String[] args)
 {
  Directory directory = new Directory(5);
  int number1 = directory.getAvailableNumber();
  System.out.println(number1);
  int number2 = directory.getAvailableNumber();
  System.out.println(number2);
  System.out.println(directory.isAvailable(3));
  int number3 = directory.getAvailableNumber();
  System.out.println(number3);
  System.out.println(directory.isAvailable(3));
  int number4 = directory.getAvailableNumber();
  System.out.println(directory.isAvailable(3));
  System.out.println(number4);
  int number5 = directory.getAvailableNumber();
  System.out.println(number5);
  int number6 = directory.getAvailableNumber();
  System.out.println(number6);
  int number7 = directory.getAvailableNumber();
  System.out.println(number7);
  directory.release(3);
  System.out.println(directory.isAvailable(3));
  int number8 = directory.getAvailableNumber();
  System.out.println(number8);
  System.out.println(directory.isAvailable(3));
 }

 private static class Directory
 {
  Node[] nodes;
  Node head;

  public Directory(int maxNumbers)
  {
   nodes = new Node[maxNumbers];
   for (int i = 0; i < maxNumbers; ++i)
   {
    nodes[i] = new Node(i);
    if (i != 0)
    {
     nodes[i].previous = nodes[i - 1];
     nodes[i - 1].next = nodes[i];
    }
   }
   nodes[maxNumbers - 1].next = nodes[0];
   nodes[0].previous = nodes[maxNumbers - 1];
   head = nodes[0];
  }

  public int getAvailableNumber()
  {
   if (head == null)
    return -1;
   int temp = head.number;
   head.available = false;
   if (head.next == head)
    head = null;
   else
   {
    head.previous.next = head.next;
    head.next.previous = head.previous;
    head = head.next;
   }
   return temp;
  }

  public boolean isAvailable(int number)
  {
   return nodes[number].available;
  }

  public void release(int number)
  {
   if (nodes[number].available == false)
   {
    nodes[number].available = true;
    if (head == null)
    {
     head = nodes[number];
     nodes[number].next = nodes[number];
     nodes[number].previous = nodes[number];
    } else
    {
     nodes[number].next = head.next;
     nodes[number].previous = head;
     head.next.previous = nodes[number];
     head.next = nodes[number];
    }
   }
  }

  private static class Node
  {
   boolean available = true;
   Node next;
   Node previous;
   int number;

   public Node(int number)
   {
    this.number = number;
   }
  }
 }

}