Вот работающий сплит


pair <pnode, pnode> split (pnode t, int k)
{
	if (t == nullptr)
	{
		return make_pair (nullptr, nullptr);
	}
	t -> relax ();
	int left_size = (t -> left == nullptr) ? 0 : t -> left -> size;
	if (left_size >= k)
	{
		pair <pnode, pnode> temp = split (t -> left, k);
		pnode r = new node (t -> value, temp.second, t -> right);
		return make_pair (temp.first, r);
	}
	else
	{
		pair <pnode, pnode> temp =
		    split (t -> right, k - (left_size + 1));
		pnode l = new node (t -> value, t -> left, temp.first);
		return make_pair (l, temp.second);
	}
}


вот неработающий

int getsizee (node* t)
{
    if(t == NULL)
        return 0;
    else
        return t->size;
}
pair <pnode, pnode> split(pnode v, int key) {
    if (v == nullptr){
        pair <node*, node*> p = make_pair(v,v);
        return p;
    }
    v -> relax ();
    int left_size = getsizee (v ->left);
    if ( left_size >= key) {
        pair <pnode, pnode> lr = split(v->left, key);
        v->left = lr.second;
        v->recalc();
        lr.second = v;
        return lr;
    }
    else {
        pair <pnode, pnode> lr = split(v->right, key - left_size - 1);
        v->right = lr.first;
        v->recalc();
        lr.first = v;
        return lr;
    }
}