iterative

    void build(const vector<T>& a)
    {
        int n = a.size();
        for (int i = 0; i < n; ++i)
            tree[n + i].query = a[i];

        for (int i = n - 1; i > 0; --i)
            tree[i].query = query(tree[2 * i].query, tree[2 * i + 1]);
    }