Submission #28445829

---

Source Code Expand

#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
// using mint = modint998244353;
using mint = modint1000000007;
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define rep2(i,k,n) for (int i = (k); i < (n); ++i)
using namespace std;
using ll = long long;
// using P = pair<ll,ll>;
using P = pair<int,int>;
using vvi = vector<vector<int>>;
using vi = vector<int>;
using vvl = vector<vector<ll>>;

// const ll INF = (ll)1e18;
// const int INF = (int)1e9+7;
template<typename T>
void chmin(T &a, T b) { a = min(a, b); }
template<typename T>
void chmax(T &a, T b) { a = max(a, b); }

template<typename T>
void print(vector<T> v) {
    int n = v.size();
    rep(i,n) {
        if (i == 0) cout << v[i].val();
        else cout << ' ' << v[i].val();
    }
    cout << endl;
}

void solve() {
    int n;
    cin >> n;
    vector<ll> a(n+1);
    rep(i,n) cin >> a[i+1];

    vector<vector<mint>> dp(n+1, vector<mint>(2, 0)), mul(n+1, vector<mint>(2, 0));
    dp[1][0] = a[1];
    mul[1][0] = 1;
    rep2(i,2,n+1) {
        mul[i][0] = mul[i-1][0] + mul[i-1][1];
        mul[i][1] = mul[i-1][0];
        dp[i][0] = dp[i-1][0] + dp[i-1][1] + mul[i][0] * a[i];
        dp[i][1] = dp[i-1][0] - mul[i][1] * a[i];
    }

    cout << (dp[n][0] + dp[n][1]).val() << endl;
}

int main() {
    solve();
    return 0;
}

Submission Info

Judge Result