Submission #3398319

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Source Code Expand

#ifndef BZ
#pragma GCC optimize "Ofast"
#endif
#include <bits/stdc++.h>
using namespace std;
using ull = uint64_t;
using ll = int64_t;
using ld = long double;

const int MOD = 1e9 + 7;

ll pw(ll a, ll b) {
    if (!b) {
        return 1;
    }

    ll v = pw(a, b / 2);
    v = (v * v) % MOD;

    if (b & 1) {
        v = (v * a) % MOD;
    }

    return  v;
}

const int MAXN = 100228;
ll rev[MAXN];

int main() {
#ifdef BZ
    freopen("input.txt", "r", stdin);
#endif
    ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); cout.setf(ios::fixed); cout.precision(20);
    int n;
    cin >> n;
    ll ans = 0;
    ll cur = 0;
    ll fact = 1;

    for (int i = 1; i <= n; ++i) {
        rev[i] = pw(i, MOD - 2);
        cur = (cur + rev[i]) % MOD;
        fact *= i;
        fact %= MOD;
    }

    for (int i = 0; i < n; ++i) {
        ll x;
        cin >> x;
        ans = (ans + x * cur) % MOD;
        cur = (cur - rev[n - i] + rev[i + 2] + MOD) % MOD;
    }

    cout << (ans * fact) % MOD << "\n";
}
 

Submission Info

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