Monistus

#include <bits/stdc++.h>
using namespace std;
 
int main() {
    string s, ans = "";
    cin >> s;
    stack<int> st, temp1, temp2;
    for (int i = (int) s.size()-1; i >= 0; --i)
        st.push(s[i]);
    
    
    while (st.size()) {
        int u = st.top(); st.pop();
        if (!isdigit(u))
            ans += u;
        else {
            int d = u-'0';
            for (int i = 0; i < d; ++i) {
                int t = st.top(); st.pop();
                temp1.push(t); temp2.push(t);
            }
            while (temp1.size()) {
                int t = temp1.top(); temp1.pop();
                st.push(t);
            }
            while (temp2.size()) {
                int t = temp2.top(); temp2.pop();
                st.push(t);
            }
        }
    }
    
    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