Monistus

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

#define N 500000

char s[N+1];
string ans;

int main() {
	string s2;
	cin >> s2;
	int i = N - s2.size();
	strcpy(s + i, s2.c_str());
	
	for (; i < N;) {
		if ('1' <= s[i] && s[i] <= '9') {
			int n2 = s[i] - '0';
			for (int j=0; j < n2; j++) {
				s[i+1-n2+j] = s[i+1+j];
			}
			i -= n2-1;
		} else {
			ans += s[i++];
		}
	}
	cout << ans << '\n';
}

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