模板：差分数组

void build() {
    for (int i = 1; i <= n; ++i) d[i] = a[i] - a[i - 1];
}
void update(int l, int r, int x) { d[l] += x, d[r + 1] -= x; }
int query(int i) {
    int res = 0;
    for (int x = 1; x <= i; ++x) res += d[x];
    return res;
}