Deletion of node from linked list - c++

Question

I am trying to code for deletion of a node with k as data of linked list. The below program is running fine, but it is not giving desired output if we have to delete the head node. For example, if the linked list is 98->6->1 and I have to delete 98, then the output which the program is showing is 0->6->1. Except for the deletion of the head node, it is working correctly for all other cases.
Below is the c++ code for the same.
#include <bits/stdc++.h>
using namespace std;
class Node
{
public :
int data;
Node* next;
};
Node * insert(Node* head, int data) {
Node* new_node= new Node();
new_node->data = data;
new_node->next = head;
head = new_node;
return head;
}
void deleteNode(Node *head, int key)
{
Node *temp = head;
Node *prev = NULL;
if(temp!=NULL && temp->data==key){
head = temp->next;
delete temp;
}
else{
while(temp!=NULL && temp->data!=key){
prev = temp;
temp = temp->next;
}
if(temp == NULL){
return;
}
prev->next = temp->next;
delete temp;
}
}
void display(Node * head) {
while(head != NULL)
{
cout<<head->data<<" ";
head = head->next;
}
}
int main() {
Node * head = NULL;
head = insert(head, 1);
head=insert(head,6);
head=insert(head,98);
deleteNode(head,98);
display(head);
return 0;
}

Answer 1

In deleteNode(), you are passing in the head node by value, so any modification made to it is not reflected back to the caller. You need to either return the new head, like you do with Insert(), or else you need to pass in the head by reference:
void deleteNode(Node* &head, int key)

Answer 2

The mistake in your code is that the head you are passing to deleteNode function is by value so the changes made to the head function is not displayed. The head in your function is not the same head in your main function it is just the copy of that head, so to apply the same change being applied to the head in deletenode function to the original head, you have to pass the address of the head(or pass by reference).
Editing in your code-
I have applied changes in deleteNode function first five lines and the third last line when you are passing value by reference deleteNode(&head,98);
Edited code-
#include <bits/stdc++.h>
using namespace std;
class Node
{
public :
int data;
Node* next;
};
Node * insert(Node* head, int data) {
Node* new_node= new Node();
new_node->data = data;
new_node->next = head;
head = new_node;
return head;
}
void deleteNode(Node **head, int key)
{
Node *temp = *head;
Node *prev = NULL;
if(temp!=NULL && temp->data==key){
*head = temp->next;
temp->next = NULL;
delete temp;
}
else{
while(temp!=NULL && temp->data!=key){
prev = temp;
temp = temp->next;
}
if(temp == NULL){
return;
}
prev->next = temp->next;
// temp->next = NULL;
delete temp;
}
}
void display(Node * head) {
while(head != NULL)
{
cout<<head->data<<" ";
head = head->next;
}
}
int main() {
Node * head = NULL;
head = insert(head, 1);
head=insert(head,6);
head=insert(head,98);
deleteNode(&head,98);
display(head);
return 0;
}
Now if you run the above code the output you will get after deleting node of value 98 will be 6->1.
Hope you will find it helpful.