non recursive segtree upd(i) get(l, r)

struct segtree {
    vector<int> t;
    int n;

    int ZERO = 0;

    segtree(int nn) : n(nn) {
        t.assign(2 * n, ZERO);
        build();
    }

    segtree(vector<int> & a) {
        n = a.size();
        t.assign(2 * n, ZERO);
        for (int i = 0; i < n; i++) t[i + n] = a[i];
        build();
    }

    int f(int x, int y) {
        return x + y;
    }

    void build() {
        for (int i = n - 1; i > 0; --i) t[i] = f(t[i << 1], t[i << 1 | 1]);
    }

    void upd(int p, int value) {
        for (t[p += n] = value; p > 1; p >>= 1) t[p >> 1] = f(t[p], t[p ^ 1]);
    }

    int get(int l, int r) {
        int res = ZERO;
        for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
            if (l & 1) res = f(res, t[l++]);
            if (r & 1) res = f(res, t[--r]);
        }
        return res;
    }
};