Monistus

#include <iostream>
#include <unordered_map>
using namespace std;

unordered_map<string, string> memo;

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

    int loo = 0;
    while (loo < (int)s.size())
    {
        if (isdigit(s[loo]))
        {
            int cn = s[loo] - '0';

            if ((int)s.size() - loo - 1 < cn) return s;

            string soo = s.substr(loo + 1, cn);
            memo[s] = s;
            soo = Boo(soo);
            s.insert(loo + 1, soo);
            s.erase(loo, 1);
        }
        else
        {
            loo++;
        }
    }

    return s;
}

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

    string soho;
    cin >> soho;

    soho = Boo(soho);

    cout << soho;

    return 0;
}

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: RUNTIME ERROR

Test 9

Group: 2

Verdict: RUNTIME ERROR

Test 10

Group: 2

Verdict: RUNTIME ERROR

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