NC19798（区间权值 ）

感受

思路

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
const ll mod = 1e9 + 7;
const int maxn = 3e5 + 10;
int n;
ll sum[maxn], suf[maxn], a[maxn], w[maxn], dp[maxn];
int main(){
    scanf("%d", &n);
    for(int i = 1; i <= n; i++){
        scanf("%lld", &a[i]);
        sum[i] = sum[i - 1] + a[i];
    }
    for(int i = n; i >= 1; i--){
        suf[i] = suf[i + 1] + a[i];
    }
    for(int i = 1; i <= n; i++){
        scanf("%lld", &w[i]);
    }
    ll ans = 0;
    for(int i = 1; i <= n; i++){
        dp[i] = dp[i - 1] - suf[n - i + 2] + sum[n] - sum[i - 1];
        dp[i] %= mod; while(dp[i] < 0) dp[i] += mod;
        ans = ans + w[i] * dp[i] % mod; ans %= mod;
    }
    printf("%lld\n", ans);
    return 0;
}