We will see what is circular doubly linked list and why is this data structure is needed and also implement an object-based circular doubly linked list in javascript.

What is circular doubly linked list?

A circular doubly linked list is a variation of linked list in which there is no end to the list. The last element of the list will point to the first element instead of pointing to the null and the first element will point to the last element. All the nodes are connected to form a circle.

Both single linked list and doubly linked list can be used to create a circular linked list.

We have already implemented a circular single linked list and have listed its advantages and disadvantages.

Now I am sure you have a good idea about what is circular doubly linked list and why do we need it, so let us start implementing an object-based circular doubly linked list in javascript.

List of operations performed on circular doubly linked list

append(element) : Adds a new element in the list.

: Adds a new element in the list. removeAt(position) : Removes an element from the given position in the list and returns it.

: Removes an element from the given position in the list and returns it. insert(position, element) : Adds an element at the given position in the list.

: Adds an element at the given position in the list. getElementAt(position) : Returns the node at the given position in the list.

: Returns the node at the given position in the list. toString() : Joins all the elements of the list and returns it as a string.

: Joins all the elements of the list and returns it as a string. toArray() : Converts the linked list to the array and returns it.

: Converts the linked list to the array and returns it. indexOf(element) : Returns the position of the given element in the list.

: Returns the position of the given element in the list. delete(element) : Removes the given element from the list.

: Removes the given element from the list. deleteHead() : Removes the first element from the list.

: Removes the first element from the list. isPresent(element) : Returns true if element is present in the list, false otherwise.

: Returns true if element is present in the list, false otherwise. isEmpty() : Returns true if the list is empty, false otherwise.

: Returns true if the list is empty, false otherwise. size() : Returns the size of the list.

: Returns the size of the list. getHead() : Returns the whole list to iterate in forward direction.

: Returns the whole list to iterate in forward direction. getTail(): Returns the whole list to iterate in reverse direction.

Implementing a circular double linked list in javascript

We will use a Node object which will be storing the element and the reference to the next element.

function circularLinkedList() { //Node let Node = function(element) { this.element = element; this.next = null; this.prev = null; } let length = 0; let head = null; let tail = null; //Other methods go here }

Getting element at a given position

Returns the node at the given index in the list. We will use this function as a helper function to get the nodes at the specific index to create a circular double linked list.

//Get element at specific index this.getElementAt = function(index) { if(index >= 0 && index <= length){ let node = head; for(let i = 0; i < index && node != null; i++){ node = node.next; } return node; } return undefined; }

Adding an item in the circular double linked list

If the head is empty then assign the current Node to the head else add the Node as the next element.

After each insertion mark the first Node as head and last Node as the tail . And then make them point to each other.

//Append a new element this.append = function(element) { let node = new Node(element), current = head, previous; if(!head){ head = node; tail = node; }else{ node.prev = tail; tail.next = node; tail = node; } //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; length++; }

Inserting element at given position in circular double linked list

There are three different cases while inserting an element in the circular doubly linked list.

Insert at the beginning

If the current element is to be added at the start then just make all the existing elements as the next element of the current element and make the last element to point the new node.



Adding new element in the middle

It is a little more complex, we need to make the element at the current index to point to the current element and current element to point to the next element and mark it as previous of next element.



Inserting a new element at the tail.

If the node is to be added to the end then make the last element point to the new element and new element point to the first element and first element to point the last element.



//Add element at any position this.insert = function(position, element) { //Check of out-of-bound values if(position >= 0 && position <= length){ let node = new Node(element), current = head, previous, index = 0; if(position === 0){ if(!head){ head = node; tail = node; }else{ node.next = current; current.prev = node; head = node; } }else if(position === length){ current = tail; current.next = node; node.prev = current; tail = node; }else{ while(index++ < position){ previous = current; current = current.next; } node.next = current; previous.next = node; //New current.prev = node; node.prev = previous; } //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; length++; return true; }else{ return false; } }

Removing element at given position in circular linked list

This step also has three different cases which needs to be handled.

Remove from the head

If the element is to be removed from the start then just make head point to the next element of the current element and make the last element to point the new head.



Deleting a middle node from the doubly linked list

It is a little more complex process, we need to make the previous element of the element at the current index to point to the next element of the current element.



Removing an element from the tail

If the node is to be removed from the tail then we need to make the second last node to point to the head.



//Remove at any position this.removeAt = function (index) { if(index >= 0 && index < length){ let current = head; //Remove from head if(index === 0){ if(length === 1){ head = undefined; }else{ const removed = head; current = this.getElementAt(length - 1); head = head.next; current.next = head; current = removed; } }else{ //Remove at given index (middle or end) const previous = this.getElementAt(index - 1); current = previous.next; previous.next = current.next; } if(head){ //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; } length--; return current.element; } return undefined; }

Get the index of the element in the circular linked list

We will be returning the zero-based index for linked list elements just like arrays, If the element is not present then we will return -1 .

//Get the indexOf item this.indexOf = function(elm){ let current = head, index = -1; //If element found then return its position while(current){ if(elm === current.element){ return ++index; } index++; current = current.next; } //Else return -1 return -1; };

Check if element is present in the circular double linked list

We can store any type of data in a doubly-linked list in javascript but here we are only finding the data for String and Numeric type.

//Find the item in the list this.isPresent = (elm) => { return this.indexOf(elm) !== -1; };

Get the head of the circular list

//Get the head this.getHead = function(){ return head; }

Get the tail of the circular list

//Get the tail this.getTail = function(){ return tail; }

Delete an element from the circular linked list

To delete an element from the list we will find the index of the element and remove it.

//Delete an item from the list this.delete = (elm) => { return this.removeAt(this.indexOf(elm)); };

Delete head from the circular linked list

To delete head from the list just remove the element at index 0.

//Delete the first item from the list this.deleteHead = function(){ this.removeAt(0); }

Delete tail from the circular linked list

To delete tail from the list just remove the element at index size - 1 .

//Delete the last item from the list this.deleteHead = function(){ this.removeAt(length - 1); }

Get the circular list as a string

It is complex to print the list as a string because we have to be careful while iterating the list that we don't end up an infinite loop.

So will keep track of the elements and break the loop once elements are repeated.

//Print item of the string this.toString = function(){ let current = head, string = ''; const temp = head.element; while(current){ if(temp === current.next.element){ string += current.element + (current.next ? '

' : ''); break; } string += current.element + (current.next ? '

' : ''); current = current.next; } return string; };

Get the circular linked list as an array

//Convert list to array this.toArray = function(){ let arr = [], current = head; const temp = head.element while(current){ //Break if first element detected if(temp === current.next.element){ arr.push(current.element); break; } arr.push(current.element); current = current.next; } return arr; };

Check if circular doubly linked list is empty

//Check if list is empty this.isEmpty = function(){ return length === 0; };

Get the size of the list

//Get the size of the list this.size = function(){ return length; }

Complete code of the circular linked list in javascript.

function circularLinkedList() { //Node let Node = function(element) { this.element = element; this.next = null; this.prev = null; } let length = 0; let head = null; let tail = null; //Other methods go here //Get element at specific index this.getElementAt = function(index) { if(index >= 0 && index <= length){ let node = head; for(let i = 0; i < index && node != null; i++){ node = node.next; } return node; } return undefined; } //Append a new element this.append = function(element) { let node = new Node(element), current = head, previous; if(!head){ head = node; tail = node; }else{ node.prev = tail; tail.next = node; tail = node; } //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; length++; } //Add element at any position this.insert = function(position, element) { //Check of out-of-bound values if(position >= 0 && position <= length){ let node = new Node(element), current = head, previous, index = 0; if(position === 0){ if(!head){ head = node; tail = node; }else{ node.next = current; current.prev = node; head = node; } }else if(position === length){ current = tail; current.next = node; node.prev = current; tail = node; }else{ while(index++ < position){ previous = current; current = current.next; } node.next = current; previous.next = node; //New current.prev = node; node.prev = previous; } //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; length++; return true; }else{ return false; } } //Remove at any position this.removeAt = function (index) { if(index >= 0 && index < length){ let current = head; //Remove from head if(index === 0){ if(length === 1){ head = undefined; }else{ const removed = head; current = this.getElementAt(length - 1); head = head.next; current.next = head; current = removed; } }else{ //Remove at given index (middle or end) const previous = this.getElementAt(index - 1); current = previous.next; previous.next = current.next; } if(head){ //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; } length--; return current.element; } return undefined; } //Get the indexOf item this.indexOf = function(elm){ let current = head, index = -1; //If element found then return its position while(current){ if(elm === current.element){ return ++index; } index++; current = current.next; } //Else return -1 return -1; }; //Find the item in the list this.isPresent = (elm) => { return this.indexOf(elm) !== -1; }; //Get the head this.getHead = function(){ return head; } //Get the tail this.getTail = function(){ return tail; } //Delete an item from the list this.delete = (elm) => { return this.removeAt(this.indexOf(elm)); }; //Delete the first item from the list this.deleteHead = function(){ this.removeAt(0); } //Print item of the string this.toString = function(){ let current = head, string = ''; const temp = head.element; while(current){ if(temp === current.next.element){ string += current.element + (current.next ? '

' : ''); break; } string += current.element + (current.next ? '

' : ''); current = current.next; } return string; }; //Convert list to array this.toArray = function(){ let arr = [], current = head; const temp = head.element while(current){ //Break if first element detected if(temp === current.next.element){ arr.push(current.element); break; } arr.push(current.element); current = current.next; } return arr; }; //Check if list is empty this.isEmpty = function(){ return length === 0; }; //Get the size of the list this.size = function(){ return length; } }

Input: let cLL = new circularLinkedList(); cLL.append(10); cLL.append(20); cLL.append(30); cLL.insert(3, 50); cLL.removeAt(0); console.log(cLL.toArray()); Output: [20, 30, 50]

Making the properties and methods private with closure and IIFE (Immediately Invoked Function Expression).

let circularLinkedList = (function(){ return function circularLinkedList() { //Node let Node = function(element) { this.element = element; this.next = null; this.prev = null; } let length = 0; let head = null; let tail = null; //Other methods go here //Get element at specific index this.getElementAt = function(index) { if(index >= 0 && index <= length){ let node = head; for(let i = 0; i < index && node != null; i++){ node = node.next; } return node; } return undefined; } //Append a new element this.append = function(element) { let node = new Node(element), current = head, previous; if(!head){ head = node; tail = node; }else{ node.prev = tail; tail.next = node; tail = node; } //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; length++; } //Add element at any position this.insert = function(position, element) { //Check of out-of-bound values if(position >= 0 && position <= length){ let node = new Node(element), current = head, previous, index = 0; if(position === 0){ if(!head){ head = node; tail = node; }else{ node.next = current; current.prev = node; head = node; } }else if(position === length){ current = tail; current.next = node; node.prev = current; tail = node; }else{ while(index++ < position){ previous = current; current = current.next; } node.next = current; previous.next = node; //New current.prev = node; node.prev = previous; } //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; length++; return true; }else{ return false; } } //Remove at any position this.removeAt = function (index) { if(index >= 0 && index < length){ let current = head; //Remove from head if(index === 0){ if(length === 1){ head = undefined; }else{ const removed = head; current = this.getElementAt(length - 1); head = head.next; current.next = head; current = removed; } }else{ //Remove at given index (middle or end) const previous = this.getElementAt(index - 1); current = previous.next; previous.next = current.next; } if(head){ //Mark head's prev element as last element head.prev = tail; //Mark tail's next element as first element tail.next = head; } length--; return current.element; } return undefined; } //Get the indexOf item this.indexOf = function(elm){ let current = head, index = -1; //If element found then return its position while(current){ if(elm === current.element){ return ++index; } index++; current = current.next; } //Else return -1 return -1; }; //Find the item in the list this.isPresent = (elm) => { return this.indexOf(elm) !== -1; }; //Get the head this.getHead = function(){ return head; } //Get the tail this.getTail = function(){ return tail; } //Delete an item from the list this.delete = (elm) => { return this.removeAt(this.indexOf(elm)); }; //Delete the first item from the list this.deleteHead = function(){ this.removeAt(0); } //Print item of the string this.toString = function(){ let current = head, string = ''; const temp = head.element; while(current){ if(temp === current.next.element){ string += current.element + (current.next ? '

' : ''); break; } string += current.element + (current.next ? '

' : ''); current = current.next; } return string; }; //Convert list to array this.toArray = function(){ let arr = [], current = head; const temp = head.element while(current){ //Break if first element detected if(temp === current.next.element){ arr.push(current.element); break; } arr.push(current.element); current = current.next; } return arr; }; //Check if list is empty this.isEmpty = function(){ return length === 0; }; //Get the size of the list this.size = function(){ return length; } } }());

ES6 class based implementation of cicular doubly linked list in javascript.

//Node class Node{ constructor(elm, next = null, prev=null){ this.element = elm; this.next = next; this.prev = prev; } } class circularLinkedList{ constructor(){ this.length = 0; this.head = null; this.tail = null; } //Get element at specific index getElementAt = function(index) { if(index >= 0 && index <= this.length){ let node = this.head; for(let i = 0; i < index && node != null; i++){ node = node.next; } return node; } return undefined; } //Add new element append = function(element) { let node = new Node(element), current = this.head, previous; if(!this.head){ this.head = node; this.tail = node; }else{ node.prev = this.tail; this.tail.next = node; this.tail = node; } //Mark head's prev element as last element this.head.prev = this.tail; //Mark tail's next element as first element this.tail.next = this.head; this.length++; } //Add element insert = function(position, element) { //Check of out-of-bound values if(position >= 0 && position <= this.length){ let node = new Node(element), current = this.head, previous, index = 0; if(position === 0){ if(!this.head){ this.head = node; this.tail = node; }else{ node.next = current; current.prev = node; this.head = node; } }else if(position === this.length){ current = this.tail; current.next = node; node.prev = current; this.tail = node; }else{ while(index++ < position){ previous = current; current = current.next; } node.next = current; previous.next = node; //New current.prev = node; node.prev = previous; } //Mark head's prev element as last element this.head.prev = this.tail; //Mark tail's next element as first element this.tail.next = this.head; this.length++; return true; }else{ return false; } } //Remove element at any position removeAt = function(position){ //look for out-of-bounds value if(position > -1 && position < this.length){ let current = this.head, previous, index = 0; //Removing first item if(position === 0){ this.head = current.next; //if there is only one item, update tail //NEW if(length === 1){ this.tail = null; }else{ this.head.prev = null; } }else if(position === this.length - 1){ current = this.tail; this.tail = current.prev; this.tail.next = null; }else{ while(index++ < position){ previous = current; current = current.next; } //link previous with current's next - skip it previous.next = current.next; current.next.prev = previous; } if(this.head){ //Mark head's prev element as last element this.head.prev = this.tail; //Mark tail's next element as first element this.tail.next = this.head; } this.length--; return current.element; }else{ return null; } } //Get the indexOf item indexOf = function(elm){ let current = this.head, index = -1; //If element found then return its position while(current){ if(elm === current.element){ return ++index; } index++; current = current.next; } //Else return -1 return -1; }; //Find the item in the list isPresent = (elm) => { return this.indexOf(elm) !== -1; }; //Delete an item from the list delete = (elm) => { return this.removeAt(this.indexOf(elm)); }; //Delete first item from the list deleteHead = function(){ this.removeAt(0); } //Delete last item from the list deleteTail = function(){ this.removeAt(this.length-1); } //Print item of the string toString = function(){ let current = this.head, string = ''; //Keep track of the head const temp = this.head.element; while(current){ //If head is the next element then break if(temp === current.next.element){ string += current.element + (current.next ? '

' : ''); break; } string += current.element + (current.next ? '

' : ''); current = current.next; } return string; }; //Convert list to array toArray = function(){ let arr = [], current = this.head; //Keep track of head const temp = this.head.element while(current){ //Break if first element detected if(temp === current.next.element){ arr.push(current.element); break; } arr.push(current.element); current = current.next; } return arr; }; //Check if list is empty isEmpty = function(){ return this.length === 0; }; //Get the size of the list size = function(){ return this.length; } //Get the head getHead = function() { return this.head; } //Get the tail getTail = function() { return this.tail; } }

Making this class private with closure and IIFE

let circularLinkedList = (function(){ //Node class Node{ constructor(elm, next = null, prev=null){ this.element = elm; this.next = next; this.prev = prev; } } return class circularLinkedList{ constructor(){ this.length = 0; this.head = null; this.tail = null; } //Get element at specific index getElementAt = function(index) { if(index >= 0 && index <= this.length){ let node = this.head; for(let i = 0; i < index && node != null; i++){ node = node.next; } return node; } return undefined; } //Add new element append = function(element) { let node = new Node(element), current = this.head, previous; if(!this.head){ this.head = node; this.tail = node; }else{ node.prev = this.tail; this.tail.next = node; this.tail = node; } //Mark head's prev element as last element this.head.prev = this.tail; //Mark tail's next element as first element this.tail.next = this.head; this.length++; } //Add element insert = function(position, element) { //Check of out-of-bound values if(position >= 0 && position <= this.length){ let node = new Node(element), current = this.head, previous, index = 0; if(position === 0){ if(!this.head){ this.head = node; this.tail = node; }else{ node.next = current; current.prev = node; this.head = node; } }else if(position === this.length){ current = this.tail; current.next = node; node.prev = current; this.tail = node; }else{ while(index++ < position){ previous = current; current = current.next; } node.next = current; previous.next = node; //New current.prev = node; node.prev = previous; } //Mark head's prev element as last element this.head.prev = this.tail; //Mark tail's next element as first element this.tail.next = this.head; this.length++; return true; }else{ return false; } } //Remove element at any position removeAt = function(position){ //look for out-of-bounds value if(position > -1 && position < this.length){ let current = this.head, previous, index = 0; //Removing first item if(position === 0){ this.head = current.next; //if there is only one item, update tail //NEW if(length === 1){ this.tail = null; }else{ this.head.prev = null; } }else if(position === this.length - 1){ current = this.tail; this.tail = current.prev; this.tail.next = null; }else{ while(index++ < position){ previous = current; current = current.next; } //link previous with current's next - skip it previous.next = current.next; current.next.prev = previous; } if(this.head){ //Mark head's prev element as last element this.head.prev = this.tail; //Mark tail's next element as first element this.tail.next = this.head; } this.length--; return current.element; }else{ return null; } } //Get the indexOf item indexOf = function(elm){ let current = this.head, index = -1; //If element found then return its position while(current){ if(elm === current.element){ return ++index; } index++; current = current.next; } //Else return -1 return -1; }; //Find the item in the list isPresent = (elm) => { return this.indexOf(elm) !== -1; }; //Delete an item from the list delete = (elm) => { return this.removeAt(this.indexOf(elm)); }; //Delete first item from the list deleteHead = function(){ this.removeAt(0); } //Delete last item from the list deleteTail = function(){ this.removeAt(this.length-1); } //Print item of the string toString = function(){ let current = this.head, string = ''; //Keep track of the head const temp = this.head.element; while(current){ //If head is the next element then break if(temp === current.next.element){ string += current.element + (current.next ? '

' : ''); break; } string += current.element + (current.next ? '

' : ''); current = current.next; } return string; }; //Convert list to array toArray = function(){ let arr = [], current = this.head; //Keep track of head const temp = this.head.element while(current){ //Break if first element detected if(temp === current.next.element){ arr.push(current.element); break; } arr.push(current.element); current = current.next; } return arr; }; //Check if list is empty isEmpty = function(){ return this.length === 0; }; //Get the size of the list size = function(){ return this.length; } //Get the head getHead = function() { return this.head; } //Get the tail getTail = function() { return this.tail; } } }());

Time Complexity

# Access Search Insert Delete Average Θ(N) Θ(N) Θ(1) Θ(1) Worst O(N) O(N) O(1) O(1)

Space Complexity