Singly

Linked Lists

Objectives

 

  • Define what a Singly Linked List is
  • Compare and contrast Linked Lists with Arrays
  • Implement insertion, removal and traversal methods on Singly Linked Lists

What is a linked list?

A data structure that contains a head, tail and length property.

Linked Lists consist of nodes, and each node has a value and a pointer to another node or null

Singly Linked Lists

null

next

next

next

4

6

8

2

HEAD

TAIL

LENGTH = 4

Comparisons with Arrays

Lists 

  • Do not have indexes!
  • Connected via nodes with a next pointer
  • Random access is not allowed

Arrays

  • Indexed in order!
  • Insertion and deletion can be expensive
  • Can quickly be accessed at a specific index

Pushing

Adding a new node to the end of the Linked List!

Pushing pseudocode

Let's visualize this!

  • This function should accept a value
  • Create a new node using the value passed to the function
  • If there is no head property on the list, set the head and tail to be the newly created node
  • Otherwise set the next property on the tail to be the new node and set the tail property on the list to be the newly created node
  • Increment the length by one
  • Return the linked list

YOUR

TURN

Popping

Removing a node from the end of the Linked List!

Popping pseudocode

Let's visualize this!

  • If there are no nodes in the list, return undefined
  • Loop through the list until you reach the tail
  • Set the next property of the 2nd to last node to be null
  • Set the tail to be the 2nd to last node
  • Decrement the length of the list by 1
  • Return the value of the node removed

YOUR

TURN

Shifting

Removing a new node from the beginning of the Linked List!

Shifting pseudocode

Let's visualize this!

  • If there are no nodes, return undefined
  • Store the current head property in a variable
  • Set the head property to be the current head's next property
  • Decrement the length by 1
  • Return the value of the node removed

YOUR

TURN

Unshifting

Adding a new node to the beginning of the Linked List!

Unshifting pseudocode

Let's visualize this!

  • This function should accept a value
  • Create a new node using the value passed to the function
  • If there is no head property on the list, set the head and tail to be the newly created node
  • Otherwise set the newly created node's next property to be the current head property on the list
  • Set the head property on the list to be that newly created node
  • Increment the length of the list by 1
  • Return the linked list

YOUR

TURN

Get

Retrieving a node by it's position in the Linked List!

Get pseudocode

Let's visualize this!

  • This function should accept an index
  • If the index is less than zero or greater than or equal to the length of the list, return null
  • Loop through the list until you reach the index and return the node at that specific index

YOUR

TURN

Set

Changing the value of a node based on it's position in the Linked List

Set pseudocode

Let's visualize this!

  • This function should accept a value and an index
  • Use your get function to find the specific node.
  • If the node is not found, return false
  • If the node is found, set the value of that node to be the value passed to the function and return true

YOUR

TURN

Insert

Adding a node to the Linked List at a specific position

Insert pseudocode

Let's visualize this!

  • If the index is less than zero or greater than the length, return false
  • If the index is the same as the length, push a new node to the end of the list
  • If the index is 0, unshift a new node to the start of the list
  • Otherwise, using the get method, access the node at the index - 1
  • Set the next property on that node to be the new node
  • Set the next property on the new node to be the previous next
  • Increment the length
  • Return true

YOUR

TURN

Remove

Removing a node from the Linked List at a specific position

Remove pseudocode

Let's visualize this!

  • If the index is less than zero or greater than the length, return undefined
  • If the index is the same as the length-1, pop
  • If the index is 0, shift
  • Otherwise, using the get method, access the node at the index - 1
  • Set the next property on that node to be the next of the next node
  • Decrement the length
  • Return the value of the node removed

YOUR

TURN

REVERSE

Reversing the Linked List

in place!

13

27

32

71

HEAD

TAIL

13

27

32

71

TAIL

HEAD

REVERSING A SINGLY LINKED LIST

Reverse pseudocode

Let's visualize this!

  • Swap the head and tail
  • Create a variable called next
  • Create a variable called prev
  • Create a variable called node and initialize it to the head property
  • Loop through the list
  • Set next to be the next property on whatever node is
  • Set the next property on the node to be whatever prev is
  • Set prev to be the value of the node variable
  • Set the node variable to be the value of the next variable
  • Once you have finished looping, return the list

YOUR

TURN

Big O

of Singly Linked Lists

Insertion -   O(1)

Removal -   It depends.... O(1) or O(N)

Searching -   O(N)

Access -   O(N)

RECAP

  • Singly Linked Lists are an excellent alternative to arrays when insertion and deletion at the beginning are frequently required
  • Arrays contain a built in index whereas Linked Lists do not
  • The idea of a list data structure that consists of nodes is the foundation for other data structures like Stacks and Queues