Monistus

// speed optimization

#include <string>
#include <iostream>

int main() {
  std::string input;
  input.reserve(500000);
  std::cin >> input;
  unsigned long int index = 0;
  while (index != std::string::npos) {
    index = input.find_first_of("123456789", index);
    input.replace(index, 1, input.substr(index+1, (int) input[index]));
    index++;
  }  
  std::cout << input;
  return 0;
}

Test 1

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

Test 2

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

Test 3

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

Test 4

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

Test 5

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

Test 6

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

Test 7

Group: 1, 2

Verdict: TIME LIMIT EXCEEDED

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