fill brothers

void addBrother(node&* begin, node&* last, node* brother) {
    if (brother) {
        if (begin) {
            last->brother = brother;
            last = brother;
        } else {
            begin = last = brother;
        }
    }
}

void bfs(node* node) {
    while(node) {
        node* brothersBegin = nullptr;
        node* brothersLast = nullptr;
        for (node* item = node; item; item = item->brother) {
            addBrother(brothersBegin, brothersLast, item->left);
            addBrother(brothersBegin, brothersLast, item->right);
        }
        node = brothersBegin;
    }
}