AtCoder Beginner Contest 353 D

https://atcoder.jp/contests/abc353/tasks/abc353_d

AtCoderではあまりないですが、和の取り方を逆にすればいい問題ですね。

// Another Sigma Problem
#![allow(non_snake_case)]

const D: u64 = 998244353;


//////////////////// library ////////////////////

fn read<T: std::str::FromStr>() -> T {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).ok();
    line.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
            .map(|e| e.parse().ok().unwrap()).collect()
}


//////////////////// process ////////////////////

fn read_input() -> Vec<u64> {
    let _N: usize = read();
    let A: Vec<u64> = read_vec();
    A
}

fn mul(mut s: u64, j: usize, a: u64) -> u64 {
    let mut b = a;
    while b > 0 {
        s *= 10;
        b /= 10
    }
    (s + a * (j as u64)) % D
}

fn F(A: Vec<u64>) -> u64 {
    let N = A.len();
    let mut s: u64 = 0;
    let mut s1: u64 = 0;
    for j in 1..N {
        s1 = (s1 + A[j-1]) % D;
        s = (s + mul(s1, j, A[j])) % D
    }
    s
}

fn main() {
    let A = read_input();
    println!("{}", F(A))
}