// O(N) speed, O(N) size.
int check_missing_c(int *a, int n) {
    int *counts = calloc(n, sizeof(n));
    int rv = 0;
    for (int i=n; i--; ) {
       if (a[i] < 0 || a[i] >= n || ++counts[a[i]] != 1)
          goto EXIT;
    }

    rv = 1;

EXIT:
    free(counts);
    return rv;
}