Doubly Linked Lists

Adding to the Tail

A Python DoublyLinkedList class can implement an .add_to_tail() instance method for adding new data to the tail of the list. .add_to_tail() takes a single new_value argument. It uses new_value to create a new Node which it adds to the tail of the list.

  def add_to_tail(self, new_value):
    new_tail = Node(new_value)
    current_tail = self.tail_node

    if current_tail != None:
      current_tail.set_next_node(new_tail)
      new_tail.set_prev_node(current_tail)

    self.tail_node = new_tail

    if self.head_node == None:
      self.head_node = new_tail

Adding to the Head

A Python DoublyLinkedList class can implement an .add_to_head() instance method for adding new data to the head of the list. .add_to_head() takes a single new_value argument. It uses new_value to create a new Node which it adds to the head of the list.

  def add_to_head(self, new_value):
    new_head = Node(new_value)
    current_head = self.head_node

    if current_head != None:
      current_head.set_prev_node(new_head)
      new_head.set_next_node(current_head)

    self.head_node = new_head

    if self.tail_node == None:
      self.tail_node = new_head

Removing the Tail

A Python DoublyLinkedList class can implement a .remove_tail() instance method for removing the head of the list. .remove_tail() takes no arguments. It removes and returns the tail of the list, and sets the tail’s previous node as the new tail.

  def remove_tail(self):
    removed_tail = self.tail_node

    if removed_tail == None:
      return None

    self.tail_node = removed_tail.get_prev_node()

    if self.tail_node != None:
      self.tail_node.set_next_node(None)

    if removed_tail == self.head_node:
      self.remove_head()

    return removed_tail.get_value()

Removing the Head

A Python DoublyLinkedList class can implement a .remove_head() instance method for removing the head of the list. .remove_head() takes no arguments. It removes and returns the head of the list, and sets the head’s next node as the new head.

  def remove_head(self):
    removed_head = self.head_node

    if removed_head == None:
      return None

    self.head_node = removed_head.get_next_node()

    if self.head_node != None:
      self.head_node.set_prev_node(None)

    if removed_head == self.tail_node:
      self.remove_tail()

    return removed_head.get_value()

Removing by Value

A Python DoublyLinkedList class can implement a .remove_by_value() instance method that takes value_to_remove as an argument and returns the node that matches value_to_remove, or None if no match exists. If the node exists, .remove_by_value() removes it from the list and correctly resets the pointers of its surrounding nodes.

  def remove_by_value(self, value_to_remove):
    node_to_remove = None
    current_node = self.head_node

    while current_node != None:
      if current_node.get_value() == value_to_remove:
        node_to_remove = current_node
        break

      current_node = current_node.get_next_node()

    if node_to_remove == None:
      return None

    if node_to_remove == self.head_node:
      self.remove_head()
    elif node_to_remove == self.tail_node:
      self.remove_tail()
    else:
      next_node = node_to_remove.get_next_node()
      prev_node = node_to_remove.get_prev_node()
      next_node.set_prev_node(prev_node)
      prev_node.set_next_node(next_node)

    return node_to_remove

Constructor

A Python DoublyLinkedList class constructor should store:

A head_node property to store the head of the list
A tail_node property to store the tail of the list

The head_node and tail_node are set to None as their defaults.

class DoublyLinkedList:
  def __init__(self):
    self.head_node = None
    self.tail_node = None

Updated Node Class

Doubly linked lists in Python utilize an updated Node class that has a pointer to the previous node. This comes with additional setter and getter methods for accessing and updating the previous node.

class Node:
  def __init__(self, value, next_node=None, prev_node=None):
    self.value = value
    self.next_node = next_node
    self.prev_node = prev_node
    
  def set_next_node(self, next_node):
    self.next_node = next_node
    
  def get_next_node(self):
    return self.next_node

  def set_prev_node(self, prev_node):
    self.prev_node = prev_node
    
  def get_prev_node(self):
    return self.prev_node
  
  def get_value(self):
    return self.value

Doubly Linked List Overview

A DoublyLinkedList class in Python has the following functionality:

A constructor with head_node and tail_node properties
An .add_to_head() method to add new nodes to the head
An .add_to_tail() method to add new nodes to the tail
A .remove_head() method to remove the head node
A .remove_tail() method to remove the tail node
A .remove_by_value() method to remove a node that matches the value_to_remove passed in

class DoublyLinkedList:
  def __init__(self):
    self.head_node = None
    self.tail_node = None

  
  def add_to_head(self, new_value):
    new_head = Node(new_value)
    current_head = self.head_node

    if current_head != None:
      current_head.set_prev_node(new_head)
      new_head.set_next_node(current_head)

    self.head_node = new_head

    if self.tail_node == None:
      self.tail_node = new_head


  def add_to_tail(self, new_value):
    new_tail = Node(new_value)
    current_tail = self.tail_node

    if current_tail != None:
      current_tail.set_next_node(new_tail)
      new_tail.set_prev_node(current_tail)

    self.tail_node = new_tail

    if self.head_node == None:
      self.head_node = new_tail


  def remove_head(self):
    removed_head = self.head_node

    if removed_head == None:
      return None

    self.head_node = removed_head.get_next_node()

    if self.head_node != None:
      self.head_node.set_prev_node(None)

    if removed_head == self.tail_node:
      self.remove_tail()

    return removed_head.get_value()


  def remove_tail(self):
    removed_tail = self.tail_node

    if removed_tail == None:
      return None

    self.tail_node = removed_tail.get_prev_node()

    if self.tail_node != None:
      self.tail_node.set_next_node(None)

    if removed_tail == self.head_node:
      self.remove_head()

    return removed_tail.get_value()


  def remove_by_value(self, value_to_remove):
    node_to_remove = None
    current_node = self.head_node

    while current_node != None:
      if current_node.get_value() == value_to_remove:
        node_to_remove = current_node
        break

      current_node = current_node.get_next_node()

    if node_to_remove == None:
      return None

    if node_to_remove == self.head_node:
      self.remove_head()
    elif node_to_remove == self.tail_node:
      self.remove_tail()
    else:
      next_node = node_to_remove.get_next_node()
      prev_node = node_to_remove.get_prev_node()
      next_node.set_prev_node(prev_node)
      prev_node.set_next_node(next_node)

    return node_to_remove