Doubly

Linked Lists

OBJECTIVES

• Construct a Doubly Linked List
• Compare and contrast Doubly and Singly Linked Lists
• Implement basic operations on a Doubly Linked List

We know what lists are...

but doubly?

Almost identical to Singly Linked Lists, except every node has another pointer, to the previous node!

Doubly Linked Lists

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null

next

next

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prev

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12

9

5

14

Comparisons with

Singly Linked Lists

More memory === More Flexibility

It's almost always a tradeoff!

PUSHING

Adding a node to the end of the Doubly Linked List

Pushing pseudocode

• Create a new node with the value passed to the function
• If the head property is null set the head and tail to be the newly created node 
• If not, set the next property on the tail to be that node
• Set the previous property on the newly created node to be the tail
• Set the tail to be the newly created node
• Increment the length
• Return the Doubly Linked List

YOUR

TURN

POPPING

Removing a node from the end of the Doubly Linked List

Popping pseudocode

• If there is no head, return undefined
• Store the current tail in a variable to return later
• If the length is 1, set the head and tail to be null
• Update the tail to be the previous Node.
• Set the newTail's next to null
• Decrement the length
• Return the value removed

YOUR

TURN

SHIFTING

Removing a node from the beginning of the Doubly Linked List

Shifting pseudocode

Let's visualize this!

• If length is 0, return undefined
• Store the current head property in a variable (we'll call it old head)
• If the length is one
  • set the head to be null
  • set the tail to be null
• Update the head to be the next of the old head
• Set the head's prev property to null
• Set the old head's next to null
• Decrement the length
• Return old head

YOUR

TURN

UNSHIFTING

Adding a node to the beginning of the Doubly Linked List

Unshifting pseudocode

• Create a new node with the value passed to the function
• If the length is 0
  • Set the head to be the new node
  • Set the tail to be the new node
• Otherwise
  • Set the prev property on the head of the list to be the new node
  • Set the next property on the new node to be the head property 
  • Update the head to be the new node
• Increment the length
• Return the list

YOUR

TURN

GET

Accessing a node in a Doubly Linked List by its position

Get Pseudocode

• If the index is less than 0 or greater or equal to the length, return null
• If the index is less than or equal to half the length of the list
  • Loop through the list starting from the head and loop towards the middle
  • Return the node once it is found
• If the index is greater than half the length of the list
  • ​Loop through the list starting from the tail and loop towards the middle
  • Return the node once it is found

YOUR

TURN

SET

Replacing the value of a node to the in a Doubly Linked List

Set pseudocode

• Create a variable which is the result of the get method at the index passed to the function
  • If the get method returns a valid node, set the value of that node to be the value passed to the function
  • Return true
• Otherwise, return false

YOUR

TURN

INSERT

Adding a node in a Doubly Linked List by a certain position

Insert pseudocode

• If the index is less than zero or greater than or equal to the length return false
• If the index is 0, unshift
• If the index is the same as the length, push
• Use the get method to access the index -1
• Set the next and prev properties on the correct nodes to link everything together
• Increment the length 
• Return true

YOUR

TURN

REMOVE

Remove pseudocode

Let's visualize this!

• If the index is less than zero or greater than or equal to the length return undefined
• If the index is 0, shift
• If the index is the same as the length-1, pop
• Use the get method to retrieve the item to be removed
• Update the next and prev properties to remove the found node from the list
• Set next and prev to null on the found node
• Decrement the length
• Return the removed node.

YOUR

TURN

Reverse

Reversing a Doubly Linked List in place!

Reverse pseudocode

• Create a variable called current and set it to be the head of the list
• Create a variable called tail and set it to be the head of the list
• Loop through the list and set the next property of the current node to be the prev property of the current node
• If there is no next property, set the tail to be the head and the head to be the current variable
• Return the list

YOUR

TURN

Big O

of Doubly Linked Lists

Technically searching is O(N / 2), but that's still O(N)

RECAP!

• Doubly Linked Lists are almost identical to Singly Linked Lists except there is an additional pointer to previous nodes
• Better than Singly Linked Lists for finding nodes and can be done in half the time!
• However, they do take up more memory considering the extra pointer
• Doubly linked lists are used to implement other data structures and certain types of caches