Monistus

#include <iostream>
#include <vector>
#include <stack>
using namespace std;

void insert(stack<char>& st, string s) {
    for (int i = s.size()-1; i >= 0; --i) st.push(s[i]);
}

int main() {
    string ini; 
    cin >> ini;
    stack<char> st;
    insert(st, ini);

    string str = "";
    while (not st.empty())
    {
        char c = st.top(); st.pop();
        if (c >= 'a' and c <= 'z') str.push_back(c);
        else {
            int n = c - '0';
            string temp = "";
            while (n--) {
                temp.push_back(st.top()); st.pop();
            }
            insert(st, temp);
            insert(st, temp);
        }
    }
    cout << str << endl;
}

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