Monistus

// https://cses.fi/484/task/B
 
#include <iostream>
using namespace std;

int main() {
    string word;
    unsigned int len;
    char c;
    unsigned int n;
    string ss;

    cin >> word;
    
    len = int(word.size());
    unsigned int i = 0;

    while (true) {
        c = word[i];
        if (isdigit(c)) {
            n = c - 48;
            len += n;
            ss = word.substr(i + 1, n);
            word = word.substr(0, i) + ss + ss + word.substr(i + 1 + n);
        } else {
            if (i >= len - 1) { break; }
            i++;
        }
    }
    cout << word << endl;
    
    }

Test 1

Group: 1, 2

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string...

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: 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