Monistus

#include <bits/stdc++.h>

using namespace std;
using ull = uint64_t;
using sll = int64_t;
static const ull inf = UINT64_MAX;
static const sll infs = INT64_MAX;

using vu = vector<ull>;
using pu = pair<ull, ull>;

string next(list<string>::iterator ent, ull index)
{
	string res;
	ull l = (*ent)[index] - '0';
	res.reserve(l);
	ull cur = index;
	for (ull i = 0; i < l; ++i)
	{
		//cout << "go" << endl;
		if (cur >= ent->size() - 1)
		{
			++ent;
			cur = 0;
		}
		else
		{
			++cur;
		}
		//cout << "P" << (*ent)[cur] << endl;
		res.push_back((*ent)[cur]);
	}
	//cout << "hey" << endl;
	return res;
}

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	list<string> rope;
	getline(cin, rope.emplace_back());

	auto it = rope.begin();
	ull index = 0;
	while (it != rope.end())
	{
		char chr = (*it)[index];
		if (chr < '0' || chr > '9')
		{
			//cout << chr << endl;
			if (index >= it->size() - 1)
			{
				++it;
				index = 0;
			}
			else
			{
				++index;
			}
		}
		else
		{
			//cout << chr << endl;
			string nxt = next(it, index);
			//cout << nxt << endl;
			if (it->begin() + index + 1 < it->end())
			{
				rope.insert(next(it), {it->begin() + index + 1, it->end()});
			}
			rope.insert(next(it), nxt);
			it->erase(index);
			++it;
			index = 0;
			//auto sit = rope.begin();
			//while (sit != rope.end())
			//{
			//	cout << *sit++;
			//}
			//cout << endl;
		}
	}

	it = rope.begin();
	while (it != rope.end())
	{
		cout << *it++;
	}
	cout << 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: 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