using System;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Iterator
{
  class Program
  {
    static void Main(string[] args)
    {
      //Numbers numbers = new Numbers();
      //foreach (int n in numbers)
      //{
      //  Console.WriteLine(n);
      //}

      DoublyLinkedList<object> list = new DoublyLinkedList<object>();
      list.AddFirst("Harry");
      list.Add("Bob");
      list.Add("John");
      list.Add("Kate");
      list.Add(3.1416);

      list.Remove("John");

      foreach (var x in list.BackEnumerator())
      {
        Console.WriteLine(x);
      }
    }
  }

  class DoublyNode<T>
  {
    public T Data { get; set; }
    public DoublyNode<T> Previous { get; set; }
    public DoublyNode<T> Next { get; set; }

    public DoublyNode(T data)
    {
      Data = data;
    }
  }

  class DoublyLinkedList<T> : IEnumerable<T>
  {
    DoublyNode<T> head;
    DoublyNode<T> tail;
    int count;

    public void Add(T data)
    {
      DoublyNode<T> node = new DoublyNode<T>(data);
      if (head == null)
      {
        head = node;
      }
      else
      {
        node.Previous = tail;
        tail.Next = node;
      }
      tail = node;
      count++;
    }

    public void AddFirst(T data)
    {
      DoublyNode<T> node = new DoublyNode<T>(data);
      if (tail == null)
      {
        tail = node;
      }
      else
      {
        node.Next = head;
        head.Previous = node;
      }
      head = node;
      count++;
    }

    public bool Remove(T data)
    {
      bool flag = false;

      DoublyNode<T> current = head;

      while (current != null)
      {
        if (current.Data.Equals(data))
        {
          break;
        }
        current = current.Next;
      }

      if (current != null)
      {
        current.Next.Previous = current.Previous;
        current.Previous.Next = current.Next;
      }

      return flag;
    }

    public IEnumerable<T> BackEnumerator()
    {
      DoublyNode<T> current = tail;
      while (current != null)
      {
        yield return current.Data;
        current = current.Previous;
      }
    }

    public IEnumerator<T> GetEnumerator()
    {
      DoublyNode<T> current = head;
      while (current != null)
      {
        yield return current.Data;
        current = current.Next;
      }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
      return GetEnumerator();
    }
  }

  class Numbers
  {
    public IEnumerator GetEnumerator()
    {
      for (int i = 0; i < 6; i++)
      {
        if (i == 3)
        {
          yield break;
        }
        yield return i * i;
      }
    }
  }
}