Monistus

#include <iostream>
#include <string>

size_t len;
int8_t num;
std::string copy_part;
char c;

std::string parse(std::string& input)
{
    len = input.length();

    if (len >= 500000)
    {
        input.erase(len, std::string::npos);
        return input;
    }

    for (uint32_t i = 0; i < len; i++)
    {
        c = input.at(i);
        if (isdigit(c))
        {
            num = c - '0';
            copy_part = input.substr(i + 1, num);

            input.erase(input.begin() + i);
            input.insert(i, copy_part);

            return parse(input);
        }
    }

    return input;
}

int main()
{
    std::string input;
    input.reserve(500000);
    std::cin >> input;

    std::cout << parse(input) << std::endl;

    return 0;
}

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: WRONG ANSWER

Test 12

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 13

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 14

Group: 2

Verdict: TIME LIMIT EXCEEDED