Merge Sort for Linked Lists in JavaScript

Last Updated : 30 Mar, 2023

Prerequisite: Merge Sort for Linked Lists Merge sort is often preferred for sorting a linked list. The slow random-access performance of a linked list makes some other algorithms (such as quicksort) perform poorly, and others (such as heapsort) completely impossible.

sorting image

In this post, Merge sort for linked list is implemented using JavaScript.

Examples:

Input : 5 -> 4 -> 3 -> 2 -> 1
Output :1 -> 2 -> 3 -> 4 -> 5

Input : 10 -> 20 -> 3 -> 2 -> 1
Output : 1 -> 2 -> 3 -> 10 -> 20

Implementation:

javascript

   <script> 
    // Create Node of LinkedList  
    function Node(data) {  
            this.node = data;  
            this.next = null;  
    }  
       
    // To initialize a linkedlist  
    function LinkedList(list) {  
        this.head = list || null
    }  
       
    // Function to insert The new Node into the linkedList  
    LinkedList.prototype.insert = function(data) {  
       
            // Check if the linked list is empty  
            // so insert first node and lead head  
            // points to generic node  
            if (this.head === null)  
                this.head = new Node(data);  
       
            else {  
       
                // If linked list is not empty, insert the node  
                // at the end of the linked list  
                let list = this.head;  
                while (list.next) {  
                    list = list.next;  
                }  
       
                // Now here list pointer points to last  
                // node let’s insert out new node in it  
                list.next = new Node(data)  
            }  
    }  
       
    // Function to print linkedList  
    LinkedList.prototype.iterate = function() {  
       
            // First we will check whether out  
            // linked list is empty or node  
            if (this.head === null)  
                return null;  
       
            // If linked list is not empty we will  
            // iterate from each Node and prints  
            // it’s value store in “data” property  
       
            let list = this.head;  
       
            // we will iterate until our list variable  
            // contains the “Next” value of the last Node  
            // i.e-> null  
            while (list) {  
                document.write(list.node)  
                if (list.next)  
                    document.write(' -> ')  
                list = list.next  
            }  
    }  
       
    // Function to mergesort a linked list  
    LinkedList.prototype.mergeSort = function(list) {  
       
            if (list.next === null)  
                return list;  
       
            let count = 0;  
            let countList = list  
            let leftPart = list;  
            let leftPointer = list;  
            let rightPart = null;  
            let rightPointer = null;  
       
            // Counting the nodes in the received linkedlist  
            while (countList.next !== null) {  
                count++;  
                countList = countList.next;  
            }  
       
            // counting the mid of the linked list  
            let mid = Math.floor(count / 2)  
            let count2 = 0;  
       
            // separating the left and right part with  
            // respect to mid node in the linked list  
            while (count2 < mid) {  
                count2++;  
                leftPointer = leftPointer.next;  
            }  
       
            rightPart = new LinkedList(leftPointer.next);  
            leftPointer.next = null;  
       
            // Here are two linked list which  
            // contains the left most nodes and right  
            // most nodes of the mid node  
            return this._mergeSort(this.mergeSort(leftPart),  
                                    this.mergeSort(rightPart.head))  
    }  
       
    // Merging both lists in a sorted manner  
    LinkedList.prototype._mergeSort = function(left, right) {  
       
            // Create a new empty linked list  
            let result = new LinkedList()  
       
            let resultPointer = result.head;  
            let pointerLeft = left;  
            let pointerRight = right;  
           
                   
            // If true then add left most node value in result,  
            // increment left pointer else do the same in  
            // right linked list.  
            // This loop will be executed until pointer's of  
            // a left node or right node reached null  
            while (pointerLeft && pointerRight) {  
                let tempNode = null;  
       
                // Check if the right node's value is greater than  
                // left node's value  
                if (pointerLeft.node > pointerRight.node) {  
                    tempNode = pointerRight.node  
                    pointerRight = pointerRight.next;  
                }  
                else {  
                    tempNode = pointerLeft.node  
                    pointerLeft = pointerLeft.next;  
                }  
       
                if (result.head == null) {  
                    result.head = new Node(tempNode)  
                    resultPointer = result.head  
                }  
                else {  
                    resultPointer.next = new Node(tempNode)  
                    resultPointer = resultPointer.next  
                }  
            }  
       
            // Add the remaining elements in the last of resultant  
            // linked list  
            resultPointer.next = pointerLeft;  
            while (resultPointer.next)  
                resultPointer = resultPointer.next  
       
                resultPointer.next = pointerRight  
       
            // Result is the new sorted linked list  
            return result.head;  
    }  
       
    // Initialize the object  
    let l = new LinkedList();  
    l.insert(10)  
    l.insert(20)  
    l.insert(3)  
    l.insert(2)  
    l.insert(1)  
    // Print the linked list  
    l.iterate()  
       
    // Sort the linked list  
    l.head = LinkedList.prototype.mergeSort(l.head)  
       
    document.write('<br> After sorting : ');  
       
    // Print the sorted linked list  
    l.iterate()  
</script> 
 
  

Output

10 -> 20 -> 3 -> 2 -> 1  After sorting : 1 -> 2 -> 3 -> 10 -> 20

Complexity Analysis:

Time Complexity: O(n*log(n))
Auxiliary Space: O(n)

Javascript Program For Merge Sort Of Linked Lists

GeetanshuGulati1

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Merge Sort for Linked Lists in JavaScript

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