Monistus

#include <iostream>
#include <string>

#define MAX_LEN 500000
#define EXTRA_RESERVE 10

std::string parse(std::string& input)
{
    std::string result;
    result.reserve(MAX_LEN + EXTRA_RESERVE);
    size_t len = input.length();
    size_t i = 0;

    while (i < len)
    {
        if (len >= MAX_LEN + EXTRA_RESERVE)
        {
            result += input.substr(0, MAX_LEN);
            return result;
        }

        size_t j = i;
        while (j < len && isdigit(input[j]))
        {
            j++;
        }

        if (j > i)
        {
            int num = std::stoi(input.substr(i, j - i));
            result += input.substr(i + 1, num);
            i = j;
        }
        else
        {
            result += input[i];
            i++;
        }
    }

    result += input.substr(i);

    return result;
}

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

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

    return 0;
}

Test 1

Group: 1, 2

Verdict: WRONG ANSWER

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: WRONG ANSWER

Test 4

Group: 1, 2

Verdict: WRONG ANSWER

Test 5

Group: 1, 2

Verdict: WRONG ANSWER

Test 6

Group: 1, 2

Verdict: WRONG ANSWER

Test 7

Group: 1, 2

Verdict: WRONG ANSWER

Test 8

Group: 2

Verdict: WRONG ANSWER

Test 9

Group: 2

Verdict: WRONG ANSWER

Test 10

Group: 2

Verdict: WRONG ANSWER

Test 11

Group: 2

Verdict: ACCEPTED

Test 12

Group: 2

Verdict: WRONG ANSWER

Test 13

Group: 2

Verdict: OUTPUT LIMIT EXCEEDED

Test 14

Group: 2

Verdict: OUTPUT LIMIT EXCEEDED