Update adjacent nodes if the current node is zero in a Singly Linked List

Given a linked list. The task is to change the value of the previous and next node of a node to 0 if the current node is 0.

Examples:

Input :  2->3->4->5->0->9->0->9->NULL
Output : 2->3->4->0->0->0->0->0->NULL

Input : 0->2->3->4->0->0->NULL
Output : 0->0->3->0->0->0->NULL

Algorithm: 

1. First step is to create two pointers prev and curr. 
   The prev will point to the previous node and curr to the current node. The reason for making use of prev is that we cannot backtrack in a singly linked list.
2. if the first node is 0 then check if the next node is 0
   • If yes, then skip
   • else change the next node to -1.
3. Start iterating through the nodes of the list.
   • if the current node is 0 and the variable temp is 0 then 
     • set the previous node to -1.
     • set the temp variable to 1.
   • if the temp value is 1 and current node data is not 1 set the current node to -1
4. Repeat step 3 until end of the list is reached.
5. Traverse through the list and change the nodes with value -1 to 0.

Reason for making use of temp and -1 is that if we change the next node to 0 directly then this may lead to change the node after next node to 0 as a result of which the entire list may get changed to 0.

Below is the implementation of the above approach:

C++

// C++ program to update the adjacent
// nodes in a linked list

#include <bits/stdc++.h>
using namespace std;

// A linked list node
struct Node {
    int data;
    struct Node* next;
};

// Utility function to create a new Node
Node* newNode(int data)
{
    Node* temp = new Node;
    temp->data = data;
    temp->next = NULL;

    return temp;
}

// Function to display the linked list
void printList(Node* node)
{
    // iterate until end of the list is reached
    while (node != NULL) {
        cout << node->data << " ";

        node = node->next;
    }
}

// Function to update the adjacent
// nodes to zero
void updateAdjacent(Node* head)
{
    // Pointer to point to
    // the previous node
    Node* prev = head;

    // Pointer to the
    // current node
    Node* curr = head->next;

    // If the first node is zero and the
    // second node is not zero then change
    // the value to -1
    if (prev->data == 0 && curr->data != 0)
        curr->data = -1;

    // Temp variable to denote if the
    // current node is zero then the next
    // node will be changed to zero
    int temp = 0;

    while (curr != NULL) {

        // if the temp variable is 1 and
        // current data is not zero
        if (temp == 1 && curr->data != 0) {

            // change the data to -1
            curr->data = -1;
            temp = 0;
        }

        // if the data is zero
        if (curr->data == 0) {

            // set the temp variable to 1
            temp = 1;

            // set the previous node data to -1
            prev->data = -1;
        }

        curr = curr->next;
        prev = prev->next;
    }

    curr = head;

    // change all the nodes with -1 to 0
    while (curr != NULL) {
        if (curr->data == -1)
            curr->data = 0;

        curr = curr->next;
    }
}

// Driver code
int main()
{
    // creating the linked list
    Node* head = newNode(2);
    head->next = newNode(3);
    head->next->next = newNode(4);
    head->next->next->next = newNode(5);
    head->next->next->next->next = newNode(0);
    head->next->next->next->next->next = newNode(9);
    head->next->next->next->next->next->next = newNode(0);
    head->next->next->next->next->next->next->next = newNode(9);

    updateAdjacent(head);

    printList(head);

    return 0;
}

Java

// Java program to update the adjacent
// nodes in a linked list
class GFG
{

// A linked list node
static class Node
{
    int data;
    Node next;
}

// Utility function to create a new Node
static Node newNode(int data)
{
    Node temp = new Node();
    temp.data = data;
    temp.next = null;

    return temp;
}

// Function to display the linked list
static void printList(Node node)
{
    // iterate until end of the list is reached
    while (node != null)
    {
        System.out.print(node.data + " ");

        node = node.next;
    }
}

// Function to update the adjacent
// nodes to zero
static void updateAdjacent(Node head)
{
    // Pointer to point to
    // the previous node
    Node prev = head;

    // Pointer to the
    // current node
    Node curr = head.next;

    // If the first node is zero and the
    // second node is not zero then change
    // the value to -1
    if (prev.data == 0 && curr.data != 0)
        curr.data = -1;

    // Temp variable to denote if the
    // current node is zero then the next
    // node will be changed to zero
    int temp = 0;

    while (curr != null)
    {

        // if the temp variable is 1 and
        // current data is not zero
        if (temp == 1 && curr.data != 0)
        {

            // change the data to -1
            curr.data = -1;
            temp = 0;
        }

        // if the data is zero
        if (curr.data == 0)
        {

            // set the temp variable to 1
            temp = 1;

            // set the previous node data to -1
            prev.data = -1;
        }

        curr = curr.next;
        prev = prev.next;
    }

    curr = head;

    // change all the nodes with -1 to 0
    while (curr != null)
    {
        if (curr.data == -1)
            curr.data = 0;

        curr = curr.next;
    }
}

// Driver code
public static void main(String[] args)
{
    // creating the linked list
    Node head = newNode(2);
    head.next = newNode(3);
    head.next.next = newNode(4);
    head.next.next.next = newNode(5);
    head.next.next.next.next = newNode(0);
    head.next.next.next.next.next = newNode(9);
    head.next.next.next.next.next.next = newNode(0);
    head.next.next.next.next.next.next.next = newNode(9);

    updateAdjacent(head);

    printList(head);
}
}

/* This code contributed by PrinciRaj1992 */

Python3

# Python3 program to update the adjacent
# nodes in a linked list

# A linked list node
class Node:

    def __init__(self, data):
        self.data = data
        self.next = None

# Function to display the linked list
def printList(node):

    # iterate until end of the
    # list is reached
    while node != None:
        print(node.data, end = " ")

        node = node.next

# Function to update the adjacent
# nodes to zero
def updateAdjacent(head):

    # Pointer to point to
    # the previous node
    prev = head

    # Pointer to the
    # current node
    curr = head.next

    # If the first node is zero and the
    # second node is not zero then change
    # the value to -1
    if prev.data == 0 and curr.data != 0:
        curr.data = -1

    # Temp variable to denote if the
    # current node is zero then the next
    # node will be changed to zero
    temp = 0

    while curr != None:

        # if the temp variable is 1 and
        # current data is not zero
        if temp == 1 and curr.data != 0:

            # change the data to -1
            curr.data = -1
            temp = 0

        # if the data is zero
        if curr.data == 0:

            # set the temp variable to 1
            temp = 1

            # set the previous node data to -1
            prev.data = -1

        curr = curr.next
        prev = prev.next

    curr = head

    # change all the nodes with -1 to 0
    while curr != None:
        if curr.data == -1:
            curr.data = 0

        curr = curr.next

# Driver Code
if __name__ == "__main__":

    # creating the linked list
    head = Node(2)
    head.next = Node(3)
    head.next.next = Node(4)
    head.next.next.next = Node(5)
    head.next.next.next.next = Node(0)
    head.next.next.next.next.next = Node(9)
    head.next.next.next.next.next.next = Node(0)
    head.next.next.next.next.next.next.next = Node(9)

    updateAdjacent(head)

    printList(head)

# This code is contributed by Rituraj Jain

C#

// C# program to update the adjacent
// nodes in a linked list
using System;

class GFG
{

// A linked list node
public class Node
{
    public int data;
    public Node next;
}

// Utility function to create a new Node
static Node newNode(int data)
{
    Node temp = new Node();
    temp.data = data;
    temp.next = null;

    return temp;
}

// Function to display the linked list
static void printList(Node node)
{
    // iterate until end of the list is reached
    while (node != null)
    {
        Console.Write(node.data + " ");

        node = node.next;
    }
}

// Function to update the adjacent
// nodes to zero
static void updateAdjacent(Node head)
{
    // Pointer to point to
    // the previous node
    Node prev = head;

    // Pointer to the
    // current node
    Node curr = head.next;

    // If the first node is zero and the
    // second node is not zero then change
    // the value to -1
    if (prev.data == 0 && curr.data != 0)
        curr.data = -1;

    // Temp variable to denote if the
    // current node is zero then the next
    // node will be changed to zero
    int temp = 0;

    while (curr != null)
    {

        // if the temp variable is 1 and
        // current data is not zero
        if (temp == 1 && curr.data != 0)
        {

            // change the data to -1
            curr.data = -1;
            temp = 0;
        }

        // if the data is zero
        if (curr.data == 0)
        {

            // set the temp variable to 1
            temp = 1;

            // set the previous node data to -1
            prev.data = -1;
        }

        curr = curr.next;
        prev = prev.next;
    }

    curr = head;

    // change all the nodes with -1 to 0
    while (curr != null)
    {
        if (curr.data == -1)
            curr.data = 0;

        curr = curr.next;
    }
}

// Driver code
public static void Main(String[] args)
{
    // creating the linked list
    Node head = newNode(2);
    head.next = newNode(3);
    head.next.next = newNode(4);
    head.next.next.next = newNode(5);
    head.next.next.next.next = newNode(0);
    head.next.next.next.next.next = newNode(9);
    head.next.next.next.next.next.next = newNode(0);
    head.next.next.next.next.next.next.next = newNode(9);

    updateAdjacent(head);

    printList(head);
}
}

// This code contributed by Rajput-Ji

Javascript

<script>
// javascript program to update the adjacent
// nodes in a linked list    // A linked list node
     class Node {

            constructor(val) {
                this.data = val;
                this.next = null;
            }

    }

    // Utility function to create a new Node
    function newNode(data) {
var temp = new Node();
        temp.data = data;
        temp.next = null;

        return temp;
    }

    // Function to display the linked list
    function printList(node) {
        // iterate until end of the list is reached
        while (node != null) {
            document.write(node.data + " ");

            node = node.next;
        }
    }

    // Function to update the adjacent
    // nodes to zero
    function updateAdjacent(head) {
        // Pointer to point to
        // the previous node
var prev = head;

        // Pointer to the
        // current node
var curr = head.next;

        // If the first node is zero and the
        // second node is not zero then change
        // the value to -1
        if (prev.data == 0 && curr.data != 0)
            curr.data = -1;

        // Temp variable to denote if the
        // current node is zero then the next
        // node will be changed to zero
        var temp = 0;

        while (curr != null) {

            // if the temp variable is 1 and
            // current data is not zero
            if (temp == 1 && curr.data != 0) {

                // change the data to -1
                curr.data = -1;
                temp = 0;
            }

            // if the data is zero
            if (curr.data == 0) {

                // set the temp variable to 1
                temp = 1;

                // set the previous node data to -1
                prev.data = -1;
            }

            curr = curr.next;
            prev = prev.next;
        }

        curr = head;

        // change all the nodes with -1 to 0
        while (curr != null) {
            if (curr.data == -1)
                curr.data = 0;

            curr = curr.next;
        }
    }

    // Driver code

        // creating the linked list
var head = newNode(2);
        head.next = newNode(3);
        head.next.next = newNode(4);
        head.next.next.next = newNode(5);
        head.next.next.next.next = newNode(0);
        head.next.next.next.next.next = newNode(9);
        head.next.next.next.next.next.next = newNode(0);
        head.next.next.next.next.next.next.next = newNode(9);

        updateAdjacent(head);

        printList(head);

// This code is contributed by todaysgaurav
</script>

Output

2 3 4 0 0 0 0 0 

Time Complexity: O(n) where n is no of nodes in given linked list