Monistus

#include <bits/stdc++.h>

#define F0R(n) for (int i=0; i<n; i++)
#define FOR(from, to) for (int i = from; i < to; i++)
#define R0F(n) for (int i = n; i>=0; --i)
#define ROF(from, to) for (int i = from; i >= to; --i)

#define ll long long
#define mod (int) (1e9 + 7)
#define vi vector<int>
#define vl vector<long>
#define vc vector<char>
#define pii pair<int, int>
#define pci pair<char, int>
#define pil pair <int, long>
#define YES cout << "YES\n";
#define NO cout << "NO\n";
using namespace std;

void print(vector<char> v) {
	for (auto it = v.begin(); it != v.end(); ++it)
		cout << *it << " ";
	cout << "\n";
}

vector<vi> adj(int(1e5+1), vi());
vi vis(int(1e5+1));

void soln() {
    int n, maxN = 500001;
    string s, ans = "";
    cin >> s;
    n = s.size();

    vc v(maxN, '*');
    F0R (n)
        v[maxN-n+i] = s[i];

    int i = maxN-n;
    while (i < maxN) {
        if ('a' <= v[i] && v[i] <= 'z')
            ans += v[i];
        else {
            int x = v[i] - '0';
            for (int j = i+1; j < i+x+1; j++)
                v[j-x] = v[j];
            i -= x;
        }
        ++i;
        //cout << ans << endl;
        //print(v);
    }
    cout << ans;
}

int main() {
    int tc=1;
    //cin >> 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: 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