int check_missing_a2(int *a, int n)
{
    for (int i = 0; i < n; ++i)
    {
        while (i != a[i])
        {
            if (a[i] < i || a[i] >= n)
                return 0;

            swap(&a[i], &a[a[i]]);
        }
    }

    return 1;
}