Monistus

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

string dupl(string s) {
    string ret = "", d, x;
    int len = s.length(), n;
    FOR (len) {
        if ('a' <= s[i]) {
            ret += s[i];
            continue;
        }
        else if (i == len-1) {
            ret += s[i];
            continue;
        }
        else {
            d = s.substr(i+1, s[i]-'0');
            d += d;
            x = dupl(d);
            n = x.length();
            if (x[n-1]-'0' < 10) {
                ret += x.substr(0, n-1);
                ret += dupl("" + (x[n-1]-'0') + s.substr(i+1+(s[i]-'0'), x[n-1]-'0'));
                //cout << "\tret = " << ret;
                //ret += s.substr(i+1+(s[i]-'0'), x[n-1]-'0') + s.substr(i+1+(s[i]-'0'), x[n-1]-'0');
                i += (x[n-1]-'0')-1;
            }
            else
                ret += x; //cout << "\tret = " << ret;
            i += (s[i]-'0');
            continue;
        }
    }
    //cout << ret << endl;
    return ret;
}

int soln() {
	/*ios_base::sync_with_stdio(false);
	cin.tie(NULL);*/
	string s;
	cin >> s;
	string ans = dupl(s);
	cout << ans;

	return 0;
}

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: WRONG ANSWER

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: WRONG ANSWER

Test 6

Group: 1, 2

Verdict: WRONG ANSWER

Test 7

Group: 1, 2

Verdict: RUNTIME ERROR

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: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: ACCEPTED