void binary(int n, int k)
{
    static string prefix;
    if (n < k) return;
    if (n == 0) { cout << prefix << endl; return; }
    prefix.push_back('0');
    binary(n-1,k);
    prefix.pop_back();
    if (k >= 1)
    {
        prefix.push_back('1');
        binary(n-1,k-1);
        prefix.pop_back();
    }
}