# 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
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 ` `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 udate 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 vaiable 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; ` `} `

## Python3

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

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 udate the adjacent
# nodes to zero

# Pointer to point to
# the previous node

# Pointer to the
# current node

# 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 vaiable 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

# 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__”:

# This code is contributed by Rituraj Jain

Output:

```2 3 4 0 0 0 0 0
```

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