Monistus

#include <bits/stdc++.h>
using namespace std;

map<string, string> memo;

string Boo(string s)
{
    if (memo.count(s))
    {
        return memo[s];
    }    

    int i = 0;
    while (i < (int)s.size())
    {
        if (isdigit(s[i]))
        {
            int num = s[i] - '0';
            string soo = s.substr(i + 1, num);
            memo[s] = s;
            soo = Boo(soo);
            s = s.substr(0, i) + soo + soo + s.substr(i + num + 1);
        }
        else
        {
            i++;
        }
    }
    return s;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    string s;
    cin >> s;

    s = Boo(s);

    cout << s;

    return 0;
}

Test 1

Group: 1, 2

Verdict: ACCEPTED

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: RUNTIME ERROR

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

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: RUNTIME ERROR

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

Test 6

Group: 1, 2

Verdict: RUNTIME ERROR

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

Test 7

Group: 1, 2

Verdict: RUNTIME ERROR

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

Test 8

Group: 2

Verdict: RUNTIME ERROR

Test 9

Group: 2

Verdict: RUNTIME ERROR

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

Test 10

Group: 2

Verdict: RUNTIME ERROR

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

Test 11

Group: 2

Verdict: RUNTIME ERROR

Test 12

Group: 2

Verdict: RUNTIME ERROR

Test 13

Group: 2

Verdict: RUNTIME ERROR

Test 14

Group: 2

Verdict: RUNTIME ERROR