Submission #23365143

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Source Code Expand

#include <bits/stdc++.h>
#include <atcoder/all>
#define endl "\n"
using namespace std;
typedef long long ll;
typedef pair<ll, ll> l_l;
typedef pair<int, int> i_i;
template<class T>
inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

const long long INF = 1e18;
//const ll mod = 1000000007;
using mint = atcoder::modint1000000007;
mint dp[100010][2];
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    mint ans = 0;
    ll N;
    cin >> N;
    dp[1][1] = 1;
    dp[1][0] = 1;
    for(int i = 2; i <= N; i++) {
        dp[i][0] = dp[i-1][1];
        dp[i][1] = dp[i-1][0] + dp[i-1][1];
        //cerr << dp[i][0].val() << " " << dp[i][1].val() << endl;
    }
    dp[0][1] = 1;
    for(int i = 0; i < N; i++) {
        ll A;
        cin >> A;
        if(i == 0) {
            ans += A * (dp[N-1][1] + dp[N-1][0]);
        } else {
            ans += A * (dp[i-1][1] + dp[i-1][0]) * (dp[N-1-i][1] + dp[N-1-i][0]);
            ans -= A * dp[i-1][1] * dp[N-1-i][1];
        }
    }
    cout << ans.val() << endl;
    return 0;
}

Submission Info

Judge Result