int GetClosestValue(Node *root, int k)
{
    if (!root)
        throw exception();

    int closest = root->value;
    Node *tmp = root;

    while (tmp)
    {
        // check if we found new closest value
        if (std::abs(tmp->value - k) < std::abs(closest - k))
            closest = tmp->value;

        // check if we can return early
        if (k == closest)
            break;

        // decide if we should left or right
        if (k < tmp->value)
            tmp = tmp->left;
        else
            tmp = tmp->right;
    }
    return closest;
}