Javascript Program For Swapping Nodes In A Linked List Without Swapping Data

Last Updated : 30 Mar, 2022

Given a linked list and two keys in it, swap nodes for two given keys. Nodes should be swapped by changing links. Swapping data of nodes may be expensive in many situations when data contains many fields.

It may be assumed that all keys in the linked list are distinct.

Examples:

Input : 10->15->12->13->20->14,  x = 12, y = 20
Output: 10->15->20->13->12->14

Input : 10->15->12->13->20->14,  x = 10, y = 20
Output: 20->15->12->13->10->14

Input : 10->15->12->13->20->14,  x = 12, y = 13
Output: 10->15->13->12->20->14

This may look a simple problem, but is an interesting question as it has the following cases to be handled.

x and y may or may not be adjacent.
Either x or y may be a head node.
Either x or y may be the last node.
x and/or y may not be present in the linked list.

How to write a clean working code that handles all the above possibilities.

The idea is to first search x and y in the given linked list. If any of them is not present, then return. While searching for x and y, keep track of current and previous pointers. First change next of previous pointers, then change next of current pointers.

Below is the implementation of the above approach.

Javascript

<script>
// JavaScript program to swap two 
// given nodes of a linked list
class Node 
{
    constructor(val) 
    {
        this.data = val;
        this.next = null;
    }
}
 
// Head of list
var head; 
 
/* Function to swap Nodes x and y in 
   linked list by changing links */
function swapNodes(x, y) 
{
    // Nothing to do if x and y 
    // are same
    if (x == y)
        return;
 
    // Search for x (keep track of 
    prevX and CurrX)
    var prevX = null, currX = head;
    while (currX != null && 
           currX.data != x) 
    {
        prevX = currX;
        currX = currX.next;
    }
 
    // Search for y (keep track of 
    // prevY and currY)
    var prevY = null, currY = head;
    while (currY != null && 
           currY.data != y) 
    {
        prevY = currY;
        currY = currY.next;
    }
 
    // If either x or y is not present, 
    // nothing to do
    if (currX == null || currY == null)
        return;
 
    // If x is not head of linked list
    if (prevX != null)
        prevX.next = currY;
    else
       
        // make y the new head
        head = currY;
 
    // If y is not head of linked list
    if (prevY != null)
        prevY.next = currX;
    else
  
        // make x the new head
        head = currX;
 
    // Swap next pointers
    var temp = currX.next;
    currX.next = currY.next;
    currY.next = temp;
}
 
// Function to add Node at beginning 
// of list
function push(new_data) 
{
    // 1. alloc the Node and put the data
    var new_Node = new Node(new_data);
 
    // 2. Make next of new Node as head
    new_Node.next = head;
 
    // 3. Move the head to point to new Node 
    head = new_Node;
}
 
// This function prints contents of 
// linked list starting from the 
// given Node
function printList() 
{
    var tNode = head;
    while (tNode != null) 
    {
        document.write(tNode.data + " ");
        tNode = tNode.next;
    }
}
 
// Driver code
/* The constructed linked list is:
   1->2->3->4->5->6->7 */
push(7);
push(6);
push(5);
push(4);
push(3);
push(2);
push(1);
 
document.write(
         "Linked list before calling swapNodes()<br/> ");
printList();
swapNodes(4, 3);
document.write(
         "<br/> Linked list after calling swapNodes() <br/>");
printList();
// This code is contributed by todaysgaurav
</script>

Output:

Linked list before calling swapNodes() 1 2 3 4 5 6 7 
Linked list after calling swapNodes() 1 2 4 3 5 6 7

Time Complexity: O(n)

Auxiliary Space: O(1)

Optimizations: The above code can be optimized to search x and y in single traversal. Two loops are used to keep program simple.

Simpler approach:

Javascript

<script>
// Javascript program to swap two given 
// nodes of a linked list
// Represent a node of the singly 
// linked list
class Node 
{
    constructor(val) 
    {
        this.data = val;
        this.next = null;
    }
}
 
// Represent the head and tail of 
// the singly linked list
var head = null;
var tail = null;
 
// addNode() will add a new node 
// to the list
function addNode(data)
{
    // Create a new node
    var newNode = new Node(data);
 
    // Checks if the list is empty
    if (head == null) 
    {
        // If list is empty, both head and
        // tail will point to new node
        head = newNode;
        tail = newNode;
    } 
    else
    {
        // newNode will be added after tail 
        // such that tail's next will point 
        // to newNode
        tail.next = newNode;
 
        // newNode will become new tail 
        // of the list
        tail = newNode;
    }
}
 
// swap() will swap the given 
// two nodes
function swap(n1 , n2) 
{
    var prevNode1 = null, 
        prevNode2 = null, 
        node1 = head, node2 = head;
 
    // Checks if list is empty
    if (head == null) 
    {
        return;
    }
 
    // If n1 and n2 are equal, then
    // list will remain the same
    if (n1 == n2)
        return;
 
    // Search for node1
    while (node1 != null && 
           node1.data != n1) 
    {
        prevNode1 = node1;
        node1 = node1.next;
    }
 
    // Search for node2
    while (node2 != null && 
           node2.data != n2) 
    {
        prevNode2 = node2;
        node2 = node2.next;
    }
 
    if (node1 != null && 
        node2 != null) 
    {
        // If previous node to node1 is not 
        // null then, it will point to node2
        if (prevNode1 != null)
            prevNode1.next = node2;
        else
            head = node2;
 
        // If previous node to node2 is 
        // not null then, it will point to node1
        if (prevNode2 != null)
            prevNode2.next = node1;
        else
            head = node1;
 
        // Swaps the next nodes of node1 and node2
        var temp = node1.next;
        node1.next = node2.next;
        node2.next = temp;
    } 
    else
    {
        document.write("Swapping is not possible");
    }
}
 
// display() will display all the
// nodes present in the list
function display() 
{
    // Node current will point to head
    var current = head;
 
    if (head == null) 
    {
        document.write("List is empty");
        return;
    }
    while (current != null) 
    {
        // Prints each node by incrementing 
        // pointer
        document.write(current.data + " ");
        current = current.next;
    }
    document.write();
}
 
// Add nodes to the list
addNode(1);
addNode(2);
addNode(3);
addNode(4);
addNode(5);
addNode(6);
addNode(7);
 
document.write("Original list:<br/> ");
display();
 
// Swaps node 2 with node 5
swap(6, 1);
 
document.write(
         "<br/>List after swapping nodes: <br/>");
display();
// This code contributed by aashish1995
</script>

Output:

Linked list before calling swapNodes() 1 2 3 4 5 6 7 
Linked list after calling swapNodes() 6 2 3 4 5 1 7

Time Complexity: O(n)

Auxiliary Space: O(1)

Please refer complete article on Swap nodes in a linked list without swapping data for more details!

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