Educational Codeforces Round 47 (Rated for Div. 2) ：E. Intercity Travelling

Posted 2020-12-24 goldenfingers

题目链接：http://codeforces.com/contest/1009/problem/E

 

解题心得：

• 一个比较简单的组合数学，还需要找一些规律，自己把方向想得差不多了但是硬是找不到规律，还是看了大佬博客才知道的规律。这个题要预先处理2的n次方，不然快速幂也会TLE。
• 推荐两个大佬的博客：
• https://blog.csdn.net/u010746456/article/details/81057082
• https://blog.csdn.net/hyp1231/article/details/81061186

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6+100;
const long long MOD = 998244353;
typedef long long ll;
ll num[maxn], pow2[maxn];
ll n;

void get_pow2(){
    pow2[0] = 1;
    for(int i=1;i<=n;i++)
        pow2[i] = (pow2[i-1] * 2) % MOD;
}


void init() {
    scanf("%lld",&n);
    for(ll i=1;i<=n;i++) {
        scanf("%lld",&num[i]);
    }
    get_pow2();
}

void solve(){
    ll ans = 0;
    for(ll i=1;i<=n;i++){
        ll temp;
        if(i == n)
            temp = 1;
        else
            temp = pow2[n-i] + pow2[n-i-1]*(n-i);
        temp %= MOD;
        ans += temp*num[i];
        ans %= MOD;
    }
    printf("%lld
",ans);
}

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