int tree :: count_elems_less_k(tree_node *root, int k, int &res)
{
    int l,r;
    if(!root)
        return 0;
    l = count_elems_less_k(root->left, k, res);
    r = count_elems_less_k(root->right, k, res);
    if(l+r < k)
    {
        res+=l+r+1;
    }
    return l+r+1;
}

int tree :: f2(tree_node *root, int k)
{
    int ret=0;
    if(!root || k<1)
        return 0;
    count_elems_less_k(root, k, ret);
    return ret;
}