Monistus

use std::{io, iter};

const MAX_SIZE: usize = 5 * 10_usize.pow(5);

#[derive(Clone)]
enum CharIter<'a> {
    End(std::str::Chars<'a>),
    Chain(Box<iter::Chain<iter::Take<CharIter<'a>>, CharIter<'a>>>),
}

impl Iterator for CharIter<'_> {
    type Item = char;

    fn next(&mut self) -> Option<Self::Item> {
        match self {
            Self::End(chars) => chars.next(),
            Self::Chain(chain) => chain.next(),
        }
    }
}

fn main() {
    let mut input = String::with_capacity(MAX_SIZE);
    io::stdin().read_line(&mut input).unwrap();

    let mut result = String::with_capacity(MAX_SIZE);
    let mut chars: CharIter = CharIter::End(input.chars());
    while let Some(ch) = chars.next() {
        if let Some(n) = ch.to_digit(10) {
            chars = CharIter::Chain(Box::new(chars.clone().take(n as usize).chain(chars)))
        } else {
            result.push(ch);
        }
    }

    println!("{}", result);
}

Test 1

Group: 1, 2

Verdict: ACCEPTED

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: ACCEPTED

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: ACCEPTED

Test 6

Group: 1, 2

Verdict: ACCEPTED

Test 7

Group: 1, 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 9

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 10

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 11

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 12

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 13

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 14

Group: 2

Verdict: TIME LIMIT EXCEEDED