Monistus

#include <bits/stdc++.h>

#define forn(i, n) for(int i=0; i<n; i++)
#define FOR(i, j, n) for(int i=j; i<n; i++)
#define pb push_back
#define F first
#define S second

typedef long long ll;

const int inf = 1.01e9;
const int mod = 1e9 + 7;

using namespace std;

string s;
string fun(string s, string rs);
string monfun(string s, string rs);

int main () {
	ios_base::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	
	cin >> s;
	
	cout << monfun(s, "") << endl;
	// cout << fun(s, "") << endl;
	
	// if (fun(s, "") == monfun(s, "")) cout << "TRUE";
	// else cout << "FALSE";
}

string fun(string s, string rs) {
	if (s.size() == 0) return rs;
	
	int num = s[0]-'0';
	
	if (!(num >= 0 && num <= 9 && (ll unsigned)num < s.size())) {
		rs.pb(s[0]);
		s.erase(s.begin());
	}
	else {
		s.erase(s.begin());
		string monistus = s.substr(0, num);
		s.erase(s.begin(), s.begin()+num);
		
		//monistus = fun(monistus, "");
		
		s = monistus + monistus + s;
	}
	
	//cout << "s: " << s << "\nrs: " << rs << "\n\n";
	return fun(s, rs);
}

string monfun(string s, string rs) {
	if (s.size() == 0) return rs;
	
	int num = s[0]-'0';
	
	if (!(num >= 0 && num <= 9 && (ll unsigned)num < s.size())) {
		rs.pb(s[0]);
		s.erase(s.begin());
	}
	else {
		s.erase(s.begin());
		string monistus = s.substr(0, num);
		s.erase(s.begin(), s.begin()+num);
		
		string monistus2 = monfun(monistus, "");
		
		s = monistus2 + monistus + s;
	}
	
	//cout << "s: " << s << "\nrs: " << rs << "\n\n";
	return monfun(s, rs);
}

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