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

static constexpr size_t wlen = 1000000;
char work[wlen];

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

	string res;
	res.reserve(wlen);
	string input;
	getline(cin, input);
	char* p = work + wlen - input.size();
	memcpy(p, input.c_str(), input.size());
	while (p < work + wlen)
	{
		if (*p < '0' || *p > '9')
		{
			res.push_back(*p++);
		}
		else
		{
			ull cnt = *p - '0';
			memcpy(p - cnt + 1, p + 1, cnt);
			p -= cnt - 1;
		}
	}
	cout << res << 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