int tree :: depth(tree_node *root)
{
    if(!root)
        return 0;
    return 1+my_max(depth(root->left),depth(root->right));
}

int tree :: count_on_lvl(tree_node *root, int level)
{
    int count=0;
    if(!root)
        return 0;
    if(level == 0)
        return 1;
    count+=count_on_lvl(root->left, level-1);
    count+=count_on_lvl(root->right, level-1);
    return count;
}

int tree :: count_on_k_lvl(tree_node *root, int k)
{
    int count_lvl=depth(root), count=0;
    if(!root || k > count_lvl)
        return 0;
    count = count_on_lvl(root, k);
    return count;
}