Problem p03300 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define dbg(...)                                                               \
  do {                                                                         \
    cerr << __LINE__ << ": ";                                                  \
    dbgprint(#__VA_ARGS__, __VA_ARGS__);                                       \
  } while (0);

using namespace std;

namespace std {
template <class S, class T> struct hash<pair<S, T>> {
  size_t operator()(const pair<S, T> &p) const {
    return ((size_t)1e9 + 7) * hash<S>()(p.first) + hash<T>()(p.second);
  }
};
template <class T> struct hash<vector<T>> {
  size_t operator()(const vector<T> &v) const {
    size_t h = 0;
    for (auto i : v)
      h = h * ((size_t)1e9 + 7) + hash<T>()(i) + 1;
    return h;
  }
};
} // namespace std
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[ ";
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? " ]" : ", ");
  return os;
}
template <class T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "{ ";
  for (const auto &i : v)
    os << i << ", ";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &v) {
  os << "{";
  for (const auto &i : v)
    os << " " << i.first << ": " << i.second << ",";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
void dbgprint(const string &fmt) { cerr << endl; }
template <class H, class... T>
void dbgprint(const string &fmt, const H &h, const T &...r) {
  cerr << fmt.substr(0, fmt.find(",")) << "= " << h << " ";
  dbgprint(fmt.substr(fmt.find(",") + 1), r...);
}
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
const int inf = (int)1e9;
const double INF = 1e12, EPS = 1e-9;

string solve(const string &s) {
  int n = s.size();
  string res;
  vi idx(n);
  int a = 0, b = 0;
  rep(i, n) {
    if (s[i] == 'a')
      idx[i] = a++;
    else
      idx[i] = b++;
  }

  if (s[0] == 'a') {
    for (int i = 0; i < n;)
      if (s[i] == 'a') {
        res += "ab";
        for (int k = idx[i++]; i < n && idx[i] != k; i++)
          ;
      }
    return res;
  }

  rep(k, a) {
    string tmp;
    rep(i, n) if (idx[i] >= k) tmp += s[i];
    res = max(res, tmp);
  }
  return res;
}

int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);

  int n, k = 0;
  string s, ans, tmp;
  cin >> n >> s;
  vector<string> v;
  for (char c : s) {
    tmp += c;
    k += c == 'a' ? 1 : -1;
    if (k == 0) {
      v.pb(solve(tmp));
      tmp.clear();
    }
  }
  for (int i = v.size() - 1; i >= 0; i--) {
    if (ans < v[i] + ans)
      ans = v[i] + ans;
  }
  cout << ans << endl;

  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define dbg(...)                                                               \
  do {                                                                         \
    cerr << __LINE__ << ": ";                                                  \
    dbgprint(#__VA_ARGS__, __VA_ARGS__);                                       \
  } while (0);

using namespace std;

namespace std {
template <class S, class T> struct hash<pair<S, T>> {
  size_t operator()(const pair<S, T> &p) const {
    return ((size_t)1e9 + 7) * hash<S>()(p.first) + hash<T>()(p.second);
  }
};
template <class T> struct hash<vector<T>> {
  size_t operator()(const vector<T> &v) const {
    size_t h = 0;
    for (auto i : v)
      h = h * ((size_t)1e9 + 7) + hash<T>()(i) + 1;
    return h;
  }
};
} // namespace std
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[ ";
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? " ]" : ", ");
  return os;
}
template <class T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "{ ";
  for (const auto &i : v)
    os << i << ", ";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &v) {
  os << "{";
  for (const auto &i : v)
    os << " " << i.first << ": " << i.second << ",";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
void dbgprint(const string &fmt) { cerr << endl; }
template <class H, class... T>
void dbgprint(const string &fmt, const H &h, const T &...r) {
  cerr << fmt.substr(0, fmt.find(",")) << "= " << h << " ";
  dbgprint(fmt.substr(fmt.find(",") + 1), r...);
}
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
const int inf = (int)1e9;
const double INF = 1e12, EPS = 1e-9;

string solve(const string &s) {
  int n = s.size();
  string res;
  vi idx(n);
  int a = 0, b = 0;
  rep(i, n) {
    if (s[i] == 'a')
      idx[i] = a++;
    else
      idx[i] = b++;
  }

  if (s[0] == 'a') {
    rep(i, n) if (s[i] == 'a') {
      res += "ab";
      for (int k = idx[i++]; i < n && idx[i] != k; i++)
        ;
    }
    return res;
  }

  rep(k, a) {
    string tmp;
    rep(i, n) if (idx[i] >= k) tmp += s[i];
    res = max(res, tmp);
  }
  return res;
}

int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);

  int n, k = 0;
  string s, ans, tmp;
  cin >> n >> s;
  vector<string> v;
  for (char c : s) {
    tmp += c;
    k += c == 'a' ? 1 : -1;
    if (k == 0) {
      v.pb(solve(tmp));
      tmp.clear();
    }
  }
  for (int i = v.size() - 1; i >= 0; i--) {
    if (ans < v[i] + ans)
      ans = v[i] + ans;
  }
  cout << ans << endl;

  return 0;
}