AtCoder Beginner Contest 433 D

https://atcoder.jp/contests/abc433/tasks/abc433_d

2つ目の引数の桁数は1～10なので、A_iを10倍から10¹⁰倍してMで割った余りをカウントしておけばよいです。

// Suddenly, A Tempest
#![allow(non_snake_case)]

use std::collections::HashMap;


//////////////////// 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() -> (i64, Vec<i64>) {
    let v: Vec<i64> = read_vec();
    let M = v[1];
    let A: Vec<i64> = read_vec();
    (M, A)
}

fn num_digits(mut n: i64) -> u32 {
    let mut num: u32 = 0;
    while n > 0 {
        n /= 10;
        num += 1
    }
    num
}

fn upper_digits(A: &Vec<i64>, M: i64) -> [HashMap<i64, usize>; 11] {
    let mut m: [HashMap<i64, usize>; 11] =
                            std::array::from_fn(|_| HashMap::new());
    let mut p: i64 = 1;
    for e in 0..11 {
        for &a in A.iter() {
            let r = a * p % M;
            let en = m[e].entry(r).or_insert(0);
            *en += 1
        }
        p = p * 10 % M
    }
    m
}

fn F(M: i64, A: Vec<i64>) -> usize {
    let mut counter: usize = 0;
    let m = upper_digits(&A, M);
    for a in A {
        let e = num_digits(a);
        let r = a % M;
        let r1 = if r == 0 { 0 } else { M - r };
        if let Some(&f) = m[e as usize].get(&r1) {
            counter += f
        }
    }
    counter
}

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