Language: C++

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

// Shortcut

#define int long long
#define endl '\n'
#define eb emplace_back
#define pb push_back
#define pob pop_back
#define mp make_pair
#define upb upper_bound
#define lwb lower_bound
#define fi first
#define se second
#define For(i, l, r) for (int i = l; i < r; i++)
#define ForE(i, l, r) for (int i = l; i <= r; i++)
#define Ford(i, r, l) for (int i = r; i > l; i--)
#define FordE(i, r, l) for (int i = r; i >= l; i--)
#define Fora(i, a) for (auto i : a)

// I/O & Debug

#define PrintV(a) for (int iiii = 0; iiii < a.size(); iiii++) cout << a[iiii] << ' ';
#define PrintVl(a) for (int iiii = 0; iiii < a.size(); iiii++) cout << a[iiii] << endl;
#define PrintA(a, n) for (int iiii = 0; iiii < n; iiii++) cout << a[iiii] << ' ';
#define PrintAl(a, n) for (int iiii = 0; iiii < n; iiii++) cout << a[iiii] << endl;
#define Ptest(x) return cout << x, 0;
#define gl(s) getline(cin, s);
#define setpre(x) fixed << setprecision(x)
/*
#define debug(args...){ string _sDEB = #args; replace(_sDEB.begin(), _sDEB.end(), ',', ' '); stringstream _ssDEB(_sDEB); istream_iterator<string> _itDEB(_ssDEB); DEB(_itDEB, args); }
void DEB(istream_iterator<string> it) {}
template<typename T, typename... Args>
void DEB(istream_iterator<string> it, T a, Args... args){
    cout << *it << " = " << a << endl;
    DEB(++it, args...);
}
*/

// Functions

//#define isvowel(a) (a == 'a' || a == 'e' || a == 'i' || a == 'o' || a == 'u')
#define bend(a) a.begin(), a.end()
#define rbend(a) a.rbegin(), a.rend()
#define mset(a) memset(a, 0, sizeof(a));
//mt19937 rando(chrono::steady_clock::now().time_since_epoch().count());

// Data Structure

#define pque priority_queue
#define mts multiset
typedef long long ll;
typedef long double ld;
typedef vector <int> vi;
typedef vector <ll> vll;
typedef vector <ld> vld;
typedef vector <string> vs;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;
typedef vector <vi > vvi;
typedef vector <vll > vvll;
typedef vector <pii > vpii;
typedef vector <pll > vpll;

const int N = 3e3 + 5, mod = 1e9 + 7, inf = 1e9 + 7;

int n;
int a[2 * N];
string s, ans;
vs b, c;

signed main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    cin >> n >> s;
    int tmp = 0, x = 0, y = 0;
    For(i, 0, s.size()){
        if (s[i] == 'a'){
            a[i] = ++x;
        }
        else{
            a[i] = ++y;
        }
        if (x == y){
            string t = "";
            if (s[tmp] == 'a'){
                int z = 0;
                ForE(j, tmp, i){
                    if (z == a[j]){
                        z = 0;
                    }
                    if (!z && s[j] == 'a'){
                        t += "ab";
                        z = a[j];
                    }
                }
            }
            else{
                ForE(j, a[tmp], a[i]){
                    string u = "";
                    ForE(k, tmp, i){
                        if (a[k] >= j){
                            u += s[k];
                        }
                    }
                    t = max(t, u);
                }
            }
            b.pb(t);
            tmp = i + 1;
        }
    }
    c.resize(b.size());
    For(i, 0, b.size()){
        c[i] = b[i];
        For(j, 0, i){
            c[i] = max(c[i], c[j] + b[i]);
        }
        ans = max(ans,)
        if (c[i] > ans){
            ans = c[i];
        }
    }
    cout << ans;
}

/*
==================================+
INPUT:                            |
------------------------------    |

------------------------------    |
==================================+
OUTPUT:                           |
------------------------------    |

------------------------------    |
==================================+
*/

Compiler output:
a.cc: In function 'int main()':
a.cc:120:23: error: expected primary-expression before ')' token
  120 |         ans = max(ans,)
      |                       ^