Linked List Sample Implementation

#include <stdio.h>
#include <stdlib.h>

typedef struct node
{
    int value;
    struct node *next;
} node_t;

node_t *new_node(int value)
{
    node_t *node = (node_t *)malloc(sizeof(node_t));
    node->value = value;
    node->next = NULL;

    return node;
}

void print_list(node_t *head)
{
    node_t *head_it = head;

    while (head_it != NULL)
    {
        printf("%d -> ", head_it->value);
        head_it = head_it->next;
    }

    printf("NULL");
}

node_t *append_node(node_t *head, int value)
{
    node_t *head_it = head;

    if (head_it == NULL)
    {
        return new_node(value);
    }

    while (head_it->next != NULL)
    {
        head_it = head_it->next;
    }

    // head_it->next = NULL
    head_it->next = new_node(value);

    return head;
}

node_t *attach_node(node_t *head, int value)
{
    node_t *new_head = new_node(value);
    new_head->next = head;
    return new_head;
}

node_t *insert_at(node_t *head, size_t index, int value)
{
    node_t *head_it = head;

    if (index == 0)
    {
        return attach_node(head, value);
    }

    if (head_it == NULL)
    {
        // If list is empty
        return new_node(value);
    }

    for (size_t i = 0; i < index - 1 && head_it->next != NULL; i++)
    {
        head_it = head_it->next;
    }

    // i == index or head->next = NULL
    if (head_it->next == NULL)
    {
        // Raggiunto fine della lista
        head_it->next = new_node(value);

        return head;
    }

    // i = index

    // head -> headNext
    // head -> new_node(value)
    // new_node->next = head->next()
    node_t *link = head_it->next;
    head_it->next = new_node(value);
    head_it->next->next = link;

    return head;
}

int main(void)
{
    node_t *head = new_node(10);
    head->next = new_node(33);
    head->next->next = new_node(100);

    print_list(head);
    printf("\n\n");

    head = attach_node(head, 66);
    head = attach_node(head, 77);

    print_list(head);
    printf("\n\n");

    head = append_node(head, 123);
    head = append_node(head, 321);

    print_list(head);
    printf("\n\n");

    head = insert_at(head, 6, 777);

    print_list(head);
    printf("\n\n");
    return 0;
}