Monistus

#include <bits/stdc++.h>

#define debug(x) cout << #x << ": " << x << "\n"
#define all(x) x.begin(), x.end()

using namespace std;
using ll = long long;

const int N = 1e5;
const int inf = 1e9;
const ll LLinf = 5e18;


int main() {

    string s;
    cin >> s;
    vector<char> v;
    for(char c : s)v.push_back(c);
    reverse(all(v));
    string ans = "";
    while(!v.empty()) {
        char c = v.back();
        v.pop_back();

        if(c >= '1' && c <= '9') {
            int x = c - '0';
            int n = (int)v.size();
            vector<char> add;
            for(int i = n - x; i < n; ++i) {
                add.push_back(v[i]);
            }
            for(char y : add)v.push_back(y);
        } else ans += string(1, c);
    }
    cout << ans << "\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: ACCEPTED