List merge

#include <stdio.h>
#include <malloc.h>

typedef struct List {
    int val;
    struct List *next;
} List;

void add(List **head, const int val) {
    if (*head == NULL) {
        *head = (List *) malloc(sizeof(List));
        (*head)->next = NULL;
        (*head)->val = val;
        return;
    }
    List *current = *head;
    while (current->next) {
        current = current->next;
    }
    current->next = (List *) malloc(sizeof(List));
    current->next->next = NULL;
    current->next->val = val;
}

List *merge(List *l1, List *l2) {
    if (l1 == NULL || l2 == NULL)
        return l2;
    List *l1_t = l1;
    List *l2_t = l2;
    List *head = NULL;
    List *cur = (List *) malloc(sizeof(List));
    cur->next = NULL;
    while (l1_t || l2_t) {
        const int v1 = l1_t == NULL ? INT_MAX : l1_t->val;
        const int v2 = l2_t == NULL ? INT_MAX : l2_t->val;
        if (v1 <= v2) {
            if (!head)
                head = l1_t;
            cur->next = l1_t;
            l1_t = l1_t->next;
            cur = cur->next;
        } else {
            if (!head)
                head = l2_t;
            cur->next = l2_t;
            l2_t = l2_t->next;
            cur = cur->next;
        }
    }

    return head;
}

void print(List *head) {
    List *current = head;
    while (current) {
        printf("%d ", current->val);
        current = current->next;
    }
}

int main() {
    List *f = NULL;
    List *t = NULL;

    add(&f, -11);
    add(&f, 1);
    add(&f, 5);

    add(&t, -40);
    add(&t, 2);
    add(&t, 4);
    add(&t, 6);

    List *r = merge(f, t);
    print(r);


}