Problem p03300 (C++)

Buggy submission (status: Runtime Error):
/*
ЗАПУСКАЕМ
░ГУСЯ░▄▀▀▀▄░РАБОТЯГУ░░
▄███▀░◐░░░▌░░░░░░░
░░░░▌░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▌░░░░░▐▄▄░░░░░
░░░░▌░░░░▄▀▒▒▀▀▀▀▄
░░░▐░░░░▐▒▒▒▒▒▒▒▒▀▀▄
░░░▐░░░░▐▄▒▒▒▒▒▒▒▒▒▒▀▄
░░░░▀▄░░░░▀▄▒▒▒▒▒▒▒▒▒▒▀▄
░░░░░░▀▄▄▄▄▄█▄▄▄▄▄▄▄▄▄▄▄▀▄
░░░░░░░░░░░▌▌░▌▌░░░░░
░░░░░░░░░░░▌▌░▌▌░░░░░
░░░░░░░░░▄▄▌▌▄▌▌░░░░░
 */
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;
template <typename T1, typename T2> inline void chkmin(T1 &x, T2 y) {
  if (x > y)
    x = y;
}
template <typename T1, typename T2> inline void chkmax(T1 &x, T2 y) {
  if (x < y)
    x = y;
}
template <typename T, typename U>
inline ostream &operator<<(ostream &_out, const pair<T, U> &_p) {
  _out << _p.first << ' ' << _p.second;
  return _out;
}
template <typename T, typename U>
inline istream &operator>>(istream &_in, pair<T, U> &_p) {
  _in >> _p.first >> _p.second;
  return _in;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const vector<T> &_v) {
  if (_v.empty()) {
    return _out;
  }
  _out << _v.front();
  for (auto _it = ++_v.begin(); _it != _v.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T> inline istream &operator>>(istream &_in, vector<T> &_v) {
  for (auto &_i : _v) {
    _in >> _i;
  }
  return _in;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const set<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const multiset<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const unordered_set<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const unordered_multiset<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T, typename U>
inline ostream &operator<<(ostream &_out, const map<T, U> &_m) {
  if (_m.empty()) {
    return _out;
  }
  _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')';
  for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) {
    _out << ", (" << _it->first << ": " << _it->second << ')';
  }
  return _out;
}
template <typename T, typename U>
inline ostream &operator<<(ostream &_out, const unordered_map<T, U> &_m) {
  if (_m.empty()) {
    return _out;
  }
  _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')';
  for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) {
    _out << ", (" << _it->first << ": " << _it->second << ')';
  }
  return _out;
}
#define sz(c) (int)(c).size()
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define left left228
#define right right228
#define next next228
#define rank rank228
#define prev prev228
#define y1 y1228
#define read(FILENAME) freopen((FILENAME + ".in").c_str(), "r", stdin)
#define write(FILENAME) freopen((FILENAME + ".out").c_str(), "w", stdout)
#define files(FILENAME) read(FILENAME), write(FILENAME)
#define pb push_back
const string FILENAME = "input";
const int MAXN = 3001;

int n;
string s;
bool used[MAXN];

string get(string a) {
  vector<int> ga, gb;
  for (int i = 0; i < sz(a); i++) {
    if (a[i] == 'a') {
      ga.pb(i);
    } else {
      gb.pb(i);
    }
  }
  if (ga[0] < gb[0]) {
    string cur;
    cur = "ab";
    int last = gb[0];
    for (int i = 1; i < sz(ga); i++) {
      if (last < ga[i]) {
        cur += "ab";
        last = gb[i];
      }
    }
    return cur;
  } else {
    string opt;
    for (int i = sz(ga) - 1; i >= 0; i--) {
      memset(used, 0, sizeof(used));
      for (int j = i; j < sz(ga); j++) {
        used[ga[j]] = true;
      }
      for (int j = i; j < sz(gb); j++) {
        used[gb[j]] = true;
      }
      string b;
      for (int j = 0; j < sz(a); j++) {
        if (used[j]) {
          b += a[j];
        }
      }
      chkmax(opt, b);
    }
    return opt;
  }
}

string dp[MAXN];

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  // read(FILENAME);
  cin >> n;
  cin >> s;
  string cur;
  int bal = 0;
  vector<string> st;
  for (int i = 0; i < 2 * n; i++) {
    cur += s[i];
    if (s[i] == 'a') {
      bal++;
    } else {
      bal--;
    }
    if (bal == 0) {
      st.pb(get(cur));
      cur.clear();
    }
  }
  for (int i = sz(st) - 1; i >= 0; i--) {
    // cout << st[i] << endl;
    dp[i] = st[i];
    chkmax(dp[i], dp[i + 1]);
    chkmax(dp[i], st[i] + dp[i + 1]);
  }
  cout << dp[0] << '\n';
}


Fixed submission:
/*
ЗАПУСКАЕМ
░ГУСЯ░▄▀▀▀▄░РАБОТЯГУ░░
▄███▀░◐░░░▌░░░░░░░
░░░░▌░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▐░░░░░▐░░░░░░░
░░░░▌░░░░░▐▄▄░░░░░
░░░░▌░░░░▄▀▒▒▀▀▀▀▄
░░░▐░░░░▐▒▒▒▒▒▒▒▒▀▀▄
░░░▐░░░░▐▄▒▒▒▒▒▒▒▒▒▒▀▄
░░░░▀▄░░░░▀▄▒▒▒▒▒▒▒▒▒▒▀▄
░░░░░░▀▄▄▄▄▄█▄▄▄▄▄▄▄▄▄▄▄▀▄
░░░░░░░░░░░▌▌░▌▌░░░░░
░░░░░░░░░░░▌▌░▌▌░░░░░
░░░░░░░░░▄▄▌▌▄▌▌░░░░░
 */
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;
template <typename T1, typename T2> inline void chkmin(T1 &x, T2 y) {
  if (x > y)
    x = y;
}
template <typename T1, typename T2> inline void chkmax(T1 &x, T2 y) {
  if (x < y)
    x = y;
}
template <typename T, typename U>
inline ostream &operator<<(ostream &_out, const pair<T, U> &_p) {
  _out << _p.first << ' ' << _p.second;
  return _out;
}
template <typename T, typename U>
inline istream &operator>>(istream &_in, pair<T, U> &_p) {
  _in >> _p.first >> _p.second;
  return _in;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const vector<T> &_v) {
  if (_v.empty()) {
    return _out;
  }
  _out << _v.front();
  for (auto _it = ++_v.begin(); _it != _v.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T> inline istream &operator>>(istream &_in, vector<T> &_v) {
  for (auto &_i : _v) {
    _in >> _i;
  }
  return _in;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const set<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const multiset<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const unordered_set<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T>
inline ostream &operator<<(ostream &_out, const unordered_multiset<T> &_s) {
  if (_s.empty()) {
    return _out;
  }
  _out << *_s.begin();
  for (auto _it = ++_s.begin(); _it != _s.end(); ++_it) {
    _out << ' ' << *_it;
  }
  return _out;
}
template <typename T, typename U>
inline ostream &operator<<(ostream &_out, const map<T, U> &_m) {
  if (_m.empty()) {
    return _out;
  }
  _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')';
  for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) {
    _out << ", (" << _it->first << ": " << _it->second << ')';
  }
  return _out;
}
template <typename T, typename U>
inline ostream &operator<<(ostream &_out, const unordered_map<T, U> &_m) {
  if (_m.empty()) {
    return _out;
  }
  _out << '(' << _m.begin()->first << ": " << _m.begin()->second << ')';
  for (auto _it = ++_m.begin(); _it != _m.end(); ++_it) {
    _out << ", (" << _it->first << ": " << _it->second << ')';
  }
  return _out;
}
#define sz(c) (int)(c).size()
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define left left228
#define right right228
#define next next228
#define rank rank228
#define prev prev228
#define y1 y1228
#define read(FILENAME) freopen((FILENAME + ".in").c_str(), "r", stdin)
#define write(FILENAME) freopen((FILENAME + ".out").c_str(), "w", stdout)
#define files(FILENAME) read(FILENAME), write(FILENAME)
#define pb push_back
const string FILENAME = "input";
const int MAXN = 6001;

int n;
string s;
bool used[MAXN];

string get(string a) {
  vector<int> ga, gb;
  for (int i = 0; i < sz(a); i++) {
    if (a[i] == 'a') {
      ga.pb(i);
    } else {
      gb.pb(i);
    }
  }
  if (ga[0] < gb[0]) {
    string cur;
    cur = "ab";
    int last = gb[0];
    for (int i = 1; i < sz(ga); i++) {
      if (last < ga[i]) {
        cur += "ab";
        last = gb[i];
      }
    }
    return cur;
  } else {
    string opt;
    for (int i = sz(ga) - 1; i >= 0; i--) {
      memset(used, 0, sizeof(used));
      for (int j = i; j < sz(ga); j++) {
        used[ga[j]] = true;
      }
      for (int j = i; j < sz(gb); j++) {
        used[gb[j]] = true;
      }
      string b;
      for (int j = 0; j < sz(a); j++) {
        if (used[j]) {
          b += a[j];
        }
      }
      chkmax(opt, b);
    }
    return opt;
  }
}

string dp[MAXN];

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  // read(FILENAME);
  cin >> n;
  cin >> s;
  string cur;
  int bal = 0;
  vector<string> st;
  for (int i = 0; i < 2 * n; i++) {
    cur += s[i];
    if (s[i] == 'a') {
      bal++;
    } else {
      bal--;
    }
    if (bal == 0) {
      st.pb(get(cur));
      cur.clear();
    }
  }
  for (int i = sz(st) - 1; i >= 0; i--) {
    // cout << st[i] << endl;
    dp[i] = st[i];
    chkmax(dp[i], dp[i + 1]);
    chkmax(dp[i], st[i] + dp[i + 1]);
  }
  cout << dp[0] << '\n';
}