//simple and easy understanding code for this problem in this problem we have to first allocate the memory to the total_number_of_books and total_number_of_pages using the calloc because we have to set the data to 0 at first

int main()
{
    int total_number_of_shelves;
    scanf("%d", &total_number_of_shelves);
    
    int total_number_of_queries;
    scanf("%d", &total_number_of_queries);
    total_number_of_books = (int*)calloc(total_number_of_shelves,sizeof(int));
    total_number_of_pages = (int**)calloc(total_number_of_shelves,sizeof(int*));
    
    for(int i =0; i< total_number_of_shelves;i++){
        total_number_of_pages[i] = (int*)calloc(total_number_of_books[i],sizeof(int));
    }
    
    while (total_number_of_queries--) {
        int type_of_query;
        scanf("%d", &type_of_query);
        
        if (type_of_query == 1) {
            /*
             * Process the query of first type here.
             */

            int x, y;
            scanf("%d %d", &x, &y);
            
            total_number_of_books[x] += 1;
            
            int size = total_number_of_books[x];
            total_number_of_pages[x] = (int*)realloc(total_number_of_pages[x], sizeof(int)*size);
            total_number_of_pages[x][size-1] = y;

        } else if (type_of_query == 2) {
            int x, y;
            scanf("%d %d", &x, &y);
            printf("%d\n", *(*(total_number_of_pages + x) + y));
        } else {
            int x;
            scanf("%d", &x);
            printf("%d\n", *(total_number_of_books + x));
        }
    }

    if (total_number_of_books) {
        free(total_number_of_books);
    }
    
    for (int i = 0; i < total_number_of_shelves; i++) {
        if (*(total_number_of_pages + i)) {
            free(*(total_number_of_pages + i));
        }
    }
    
    if (total_number_of_pages) {
        free(total_number_of_pages);
    }
    
    return 0;
}

Simplest solution: Prepare required storages then Perform insert (with realloc if necessary)

Code Solution ------------------------>

int main() ........ { int total_number_of_shelves; scanf("%d", &total_number_of_shelves);

int total_number_of_queries;
scanf("%d", &total_number_of_queries);

//1. Prepare Storage
total_number_of_books = (int *)malloc(total_number_of_shelves * sizeof(int));
total_number_of_pages = (int **)malloc(total_number_of_shelves * sizeof(int*));

    //2. Init Storage
for (int i = 0; i < total_number_of_shelves; i++) 
{
    total_number_of_books[i] = 0;
    total_number_of_pages[i] = (int*)malloc(sizeof(int));
    total_number_of_pages[i][0] = 0;
}

while (total_number_of_queries--) {
    int type_of_query;
    scanf("%d", &type_of_query);

    if (type_of_query == 1) {
        /*
         * Process the query of first type here.
         */
        int x, y;
        scanf("%d %d", &x, &y);

                    //3. Insert Book
        total_number_of_books[x]++;

                    //4. Insert Page
        if(total_number_of_books[x] == 1)
        {
            total_number_of_pages[x][0] = y;
        }
        else 
        {
            total_number_of_pages[x] = realloc(total_number_of_pages[x], total_number_of_books[x]*sizeof(int));
            total_number_of_pages[x][total_number_of_books[x]-1] = y;
        }

The problem is not clear about implementation.

Here is my explanation.

// For this, before `while` loop, you need to initialize dynamic arrays (which are declared on top of the main) to store numbers of:
// - total_number_of_books
// - total_number_of_pages

// x y : (a) Insert a book (b) with y pages (c) at the end of the x^th shelf.

// (a) Insert a book
total_number_of_books[x] += 1;  
// *(total_number_of_books + x) += 1; // the same - equivalent in old books/tutorials

// (b) allocate memory (int) for the new shelf: ( void *ptr, size_t new_size );
// - total_number_of_pages just get bigger by int as we inserted a book

// (c) assign number of pages [y] to the x^th shelf

Simpler, isn't it?

Here is Dynamic Array in C solution - https://programmingoneonone.com/hackerrank-dynamic-array-in-c-solution.html

Just wanted to share this cause I found this easier

include

include

int main() { int shelfbook[100][100], count=0, count2=0; int total_number_of_shelves; scanf("%d", &total_number_of_shelves);

int total_number_of_queries;
scanf("%d", &total_number_of_queries);

while (total_number_of_queries>0)
{   
    total_number_of_queries--;
    int type_of_query;
    scanf("%d", &type_of_query);

    if (type_of_query == 1) 
    {
        int x, y; 

        for(count; count>0; count--)
        { shelfbook[x][count]=shelfbook[x][count-1];}

        scanf("%d %d", &x, &y);
        shelfbook[x][0]= y;

        count++;

    } 
    else if (type_of_query == 2) 
    {
        int x, y;
        scanf("%d %d", &x, &y);
        printf("%d",shelfbook[x][y]);
    } 
    else if (type_of_query == 3)
    {
        int x;
        scanf("%d", &x);


        for (int y=0; y<100; y++)
        {
            if (shelfbook[x][y]!=0)
            { count2++;}
        }
        printf("%d\n", count2);
    }
}

return 0;

}