Monistus

#include <iostream>
#include <string>

#define MAX_LEN 500000
#define EXTRA_RESERVE 10

uint32_t i = 0;
uint32_t len;
int8_t num;
std::string copy_part;
char c;

// BRUH...

std::string parse(std::string& input)
{
    len = input.length();
    while (i < len)
    {
        if (len >= MAX_LEN + EXTRA_RESERVE)
        {
            return input.substr(0, MAX_LEN);
        }

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

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

                len += num - 1;

                break;
            }
        }
    }

    return input;
}

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

    input.reserve(MAX_LEN + EXTRA_RESERVE);

    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: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

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