一维前缀和

const int N = 1e5+5;
using ll = long long;

ll a[N] = {0};

for (int i = 1; i <= n; i++) {
    cin >> a[i];
    a[i] += a[i-1];   // 原地构造前缀和
}

// 查询区间 [L, R] 的和
cout << a[R] - a[L-1];