Monistus

#include <iostream>
#include <string>
#include <vector>
#include <sstream>

constexpr char const* digits = "123456789";

///
/// \brief expand_recurse Expand the sting from the next leading number, recursively
/// \param str
/// \return expanded result
/// First recursively calculates the expanded form of the n following characters, then duplicates it twice
///
std::string expand_recurse(std::string str) {
    size_t next_digit;
    size_t pos = 0;
    str.reserve(50000);

    while (next_digit = str.find_first_of(digits, pos), next_digit != str.npos) {
        pos = next_digit;
        size_t const n = str[next_digit] - '0';
        auto sub = str.substr(next_digit + 1, n);
        sub += sub;

        str.replace(next_digit, 1 + n, sub, 0, 1 + n);
        str.insert(next_digit + 1 + n, sub, 1 + n);

    }

    return str;
}

int main() {
//    std::istringstream input("82silisko");
    auto& input = std::cin;

    std::string str;
    std::getline(input, str);

    auto res = expand_recurse(str);
    std::cout << res << "\n";

}

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: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: TIME LIMIT EXCEEDED