Segment Tree Implementation

int seq[MAXN], segt[4*MAXN], n;

void build (int node, int l, int r) {
    if (l == r) {
        segt[node] = seq[l];
    }else {
        build(2*node+1, l, (l+r)/2);
        build(2*node+2, (l+r)/2+1, r);
        segt[node] = segt[node*2+1] * segt[node*2+2];
    }

}

void update (int node, int l, int r, int pos, int val) {
    if (l == r) {
        seq[pos] = segt[node] = val;
    } else {
        int mid = (l+r)/2;
        if (pos <= mid) update(2*node+1, l, mid, pos, val);
        else update(2*node+2, mid+1, r, pos, val);
        segt[node] = segt[node*2+1] * segt[node*2+2];
    }
}

int query  (int node, int l, int r, int low, int up) {
    if (low > r || up < l) {
        return 1;
    }
    if (low <= l && up >= r) {
        return segt[node];
    } else {
        int mid = (l+r)/2;
        return query(node*2+1, l, mid, low, up)*query(node*2+2, mid+1, r, low, up);
    }
}