动态规划（C++）

核心是这个：

下标从 1 开始
S[i] = a[1] + a[2] + ... a[i]
a[l] + ... + a[r] = S[r] - S[l - 1]

#include <iostream>

using namespace std;

const int N = 1e5 + 5;

int n, q;
int a[N];
long long s[N]; 

int main() { 
    cin >> n >> q;
    for (int i = 1; i <= n; ++i) cin >> a[i];
    for (int i = 1; i <= n; ++i) s[i] = a[i] + s[i - 1];
    while (q --) {
        int l, r;
        cin >> l >> r;
        cout << s[r] - s[l - 1] << endl;
    }
    return 0;
}