Error in program to delete the last occurence of a an element in linked list

Question

The aim was to make a program that deleted the last occurrence of an element in a linked list Ex:-If the linked list is 23->45->23->24->23 then i look for the output 23->45->23->24 but I am getting several errors on debugging mainly on the parts temp->next!=NULL and t1->next=t->next,kindly point out the errors hindering the execution of program?

#include<stdio.h>
#include<stdlib.h>
struct node
{
    int data;
    struct node *next;
};
void insertLL(struct node *head,int data)
{
    struct node *temp;
    struct node *new=(struct node *)malloc(sizeof(struct node));
    new->data=data;
    if(head==NULL)
    {
        head=new;
    }
    else
    {
        temp=head;   
        while(temp->next!=NULL)
        {
            temp=temp->next;
        }
        temp->next=new;
    }
}
void delete(struct node *head,int data)
{
    struct node *temp=head;
    struct node *t;
    struct node *t1;
    while(temp->next!=NULL)
    {
        if(temp->next->data==data)
        {
            t1=temp;
        }
        if(temp->data==data)
        {
            t=temp;
        }
        temp=temp->next;
    }
    t1->next=t->next;
    free(t);
}
void display(struct node *head)
{
    struct node *temp1=head;
    if (head==NULL)
    {
        printf("List is empty");
    }
    else
    {
        while(temp1->next!=NULL)
        {
            printf("%d",temp1->data);
            printf("->");
            temp1=temp1->next;
        }
    }
    
}
int main()
{
    int u,k,l,n;
    struct node *start;
    while(1)
    {
        printf("Press 1 to insert element into the Linked List, press 2 to delete the last occurence of an element from a linked list, press 3 to display the linked list, press 0 to exit the program\n");
        scanf("%d",&u);
        switch(u)
        {
            case 1:
            {
                printf("Enter the data you want to insert\n");
                scanf("%d",&l);
                insertLL(start,l);
                break;
            }
            case 2:
            {
                printf("Enter the element that you want to be deleted\n");
                scanf("%d",&k);
                delete(start,k);
                break;
            }
            case 3:
            {
                display(start);
                break;
            }
            default:
            {
                printf("Invalid input\n");
            }
            
        }
        printf("Enter 1 to continue or 0 to break\n");
        scanf("%d",&n);
        if(n==1)
        {
            continue;
        }
        if(n==0)
        {
            break;
        }
    }
}

Answer 1

For starters the function insertLL is incorrect because it deals with a copy of the value of the pointer to the head node. That is changing the copy does not influence on the original object.

You need to pass the pointer to the head node by reference through pointer to it.

The function can be declared and defined the following way.

int insertLL( struct node **head, int data )
{
    struct node *new_node = malloc( sizeof( struct node ) );
    int success = new_node != NULL;

    if ( success )
    {
        new_node->data = data;
        new_node->next = NULL;

        while ( *head ) head = &( *head )->next;
  
        *head = new_node;
    }

    return success;
}

And the function can be called like

insertLL( &start, data );

or

if ( !insertLL( &start, data ) )
{
    puts( "Error: not enough memory." );
}

The function delete is also incorrect. For example it can invoke undefined behavior when it is called for an empty list due to this statement

while(temp->next!=NULL)

The function can be declared and defined the following way

void delete( struct node **head, int data )
{
    // Find the last node with the target data
    struct node **target = NULL;

    while ( *head )
    {
        if ( ( *head )->data == data )
        {
            target = head;
        }
    }

    // If the target node is found remove it from the list
    if ( target != NULL )
    {
        struct node *tmp = *target;
        *target = ( *target )->next;
        free( tmp );
    }
}

And the function display is in turn incorrect. It outputs nothing when the list contains only one node due to this statement

while(temp1->next!=NULL)

The function can be declared and defined the following way

void display( const struct node *head )
{
    for ( ; head != NULL; head = head->next )
    {
        printf( "%d -> ", head->data );
    }

    puts( "null" );
}

In main these statements

        default:
        {
            printf("Invalid input\n");
        }

and

    printf("Enter 1 to continue or 0 to break\n");
    scanf("%d",&n);
    if(n==1)
    {
        continue;
    }
    if(n==0)
    {
        break;
    }

is redundant. You already wrote to the user how to exit the loop

printf("Press 1 to insert element into the Linked List, press 2 to delete the last occurence of an element from a linked list, press 3 to display the linked list, press 0 to exit the program\n");

So the loop can look for example like

while(1)
{
    printf("Press 1 to insert element into the Linked List, press 2 to delete the last occurence of an element from a linked list, press 3 to display the linked list, press 0 to exit the program\n");

    u = -1;
    scanf("%d",&u);

    switch(u)
    {
        case 1:
        {
            //...
            break;
        }
        case 2:
        {
            //...
            break;
        }
        case 3:
        {
            //...
            break;
        }
        default:
        {
            if ( u != 0 )
            {
                printf("Invalid input\n");
            }
            break;
        }
    }

    if ( u == 0 ) break;
}