Monistus

#include <bits/stdc++.h>
#define ll long unsigned int
#define FOR(m) for (ll i=0; i<m; i++)
using namespace std;

string dupl(string s) {
    string ans = s;
    string x = "";
    bool flag = false;
    //int n = s.length();

    while (true) {
        flag = false;

        FOR (s.length()) {
            if ('0' < s[i] && s[i] <= '9' && i!=s.length()-1) {
                x = dupl(s.substr(i+1, s[i]-'0'));
                ans = s.substr(0, i) + x + x + s.substr(i+1+(s[i]-'0'));
                flag = true;
                break;
            }
        }

        s = ans;

        if (!flag) {
            return s;
            break;
        }
    }
}

void soln() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	string s, ans = "";
	cin >> s;
	ans = dupl(s);

	cout << ans;
}

int main() {
	int tc=1;
	//scanf("%d", &tc);
	while (tc--) soln();
	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: ACCEPTED

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: TIME LIMIT EXCEEDED

Test 12

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 13

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 14

Group: 2

Verdict: TIME LIMIT EXCEEDED