void BTree::delet(int k)
{
    if (!root)
    {
        cout << "Books are not available in the market\n";
        return;
    }
    root->delet(k);
    if (root->n==0)
    {
        BTreeNode *tmp = root;
        if (root->leaf)
            root = NULL;
        else
            root = root->C[0];
        delete tmp;
    }
    return;
}