Remove duplicates from an unsorted array with malloc in C Solution, output & explanation

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

int *unique_values(const int *a, size_t n, size_t *out_n) {
    bool *seen = calloc(100, sizeof *seen);          /* zeroed: nothing seen yet */
    int *out = malloc(n * sizeof *out);
    if (seen == NULL || out == NULL) {
        free(seen);
        free(out);
        return NULL;
    }
    size_t k = 0;
    for (size_t i = 0; i < n; i++)
        if (a[i] >= 0 && a[i] < 100 && !seen[a[i]]) {
            seen[a[i]] = true;
            out[k++] = a[i];
        }
    free(seen);
    *out_n = k;
    return out;
}

int main(void) {
    int a[] = {4, 7, 4, 1, 7, 9, 1, 4};
    size_t k;
    int *u = unique_values(a, 8, &k);
    if (u == NULL)
        return 1;
    for (size_t i = 0; i < k; i++)
        printf("%d ", u[i]);
    printf("(%zu unique)\n", k);
    free(u);
    return 0;
}

4 7 1 9 (4 unique)