Merge Sort for Linked Lists in JavaScript

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.
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

<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 tke 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 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 


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