Problem p03300 (C++)

Buggy submission (status: Runtime Error):
// #define _GLIBCXX_DEBUG

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

#define endl '\n'
#define lfs cout << fixed << setprecision(10)
#define ALL(a) (a).begin(), (a).end()
#define ALLR(a) (a).rbegin(), (a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i, n, m) for (ll i = (n); i < (ll)(m); i++)
#define rrep(i, n, m) for (ll i = (ll)(m)-1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9 + 7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template <typename T1, typename T2> bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return true;
  } else
    return false;
}
template <typename T1, typename T2> bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return true;
  } else
    return false;
}
ll median(ll a, ll b, ll c) {
  return a + b + c - max({a, b, c}) - min({a, b, c});
}
void ans1(bool x) {
  if (x)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void ans2(bool x) {
  if (x)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void ans3(bool x) {
  if (x)
    cout << "Yay!" << endl;
  else
    cout << ":(" << endl;
}
template <typename T1, typename T2> void ans(bool x, T1 y, T2 z) {
  if (x)
    cout << y << endl;
  else
    cout << z << endl;
}
template <typename T> void debug(vector<vector<T>> &v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    cout << v[i][0];
    for (ll j = 1; j < w; j++)
      cout spa v[i][j];
    cout << endl;
  }
};
void debug(vector<string> &v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++)
      cout << v[i][j];
    cout << endl;
  }
};
template <typename T> void debug(vector<T> &v, ll n) {
  if (n != 0)
    cout << v[0];
  for (ll i = 1; i < n; i++)
    cout spa v[i];
  cout << endl;
};
template <typename T> vector<vector<T>> vec(ll x, ll y, T w) {
  vector<vector<T>> v(x, vector<T>(y, w));
  return v;
}
ll gcd(ll x, ll y) {
  ll r;
  while (y != 0 && (r = x % y) != 0) {
    x = y;
    y = r;
  }
  return y == 0 ? x : y;
}
vector<ll> dx = {1, -1, 0, 0, 1, 1, -1, -1};
vector<ll> dy = {0, 0, 1, -1, 1, -1, 1, -1};
template <typename T> vector<T> make_v(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  return os << p.first << " " << p.second;
}
// mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());

int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res = 0, buf = 0;
  bool judge = true;
  ll n;
  cin >> n;
  string s;
  cin >> s;
  vector<ll> a, b, c;
  rep(i, 0, 2 * n) {
    if (s[i] == 'a') {
      c.PB(a.size());
      a.PB(i);
    } else {
      c.PB(b.size());
      b.PB(i);
    }
  }
  auto f = [&](string t) {
    ll nxt = 0;
    queue<ll> ch;
    stack<ll> st;
    rep(i, 0, 2 * n) {
      if (nxt == t.size())
        break;
      if (ch.front() == i) {
        ch.pop();
        if (s[i] == t[nxt]) {
          st.push(c[i]);
          nxt++;
        } else {
          ll mi = min(a[c[i]], b[c[i]]);
          while (!st.empty()) {
            ll p = st.top();
            if (max(a[p], b[p]) > mi) {
              chmin(mi, min(a[p], b[p]));
              nxt -= 2;
              st.pop();
            } else
              break;
          }
          nxt--;
        }
      } else if (min(a[c[i]], b[c[i]]) == i && s[i] == t[nxt]) {
        nxt++;
        ch.push(max(a[c[i]], b[c[i]]));
      }
    }
    // cout<<t spa nxt<<endl;
    return nxt == t.size();
  };
  string ret;
  while (1) {
    if (f(ret + 'b')) {
      ret += 'b';
    } else if (f(ret + 'a')) {
      ret += 'a';
    } else
      break;
  }
  cout << ret << endl;
  return 0;
}

Fixed submission:
// #define _GLIBCXX_DEBUG

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

#define endl '\n'
#define lfs cout << fixed << setprecision(10)
#define ALL(a) (a).begin(), (a).end()
#define ALLR(a) (a).rbegin(), (a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i, n, m) for (ll i = (n); i < (ll)(m); i++)
#define rrep(i, n, m) for (ll i = (ll)(m)-1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9 + 7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template <typename T1, typename T2> bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return true;
  } else
    return false;
}
template <typename T1, typename T2> bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return true;
  } else
    return false;
}
ll median(ll a, ll b, ll c) {
  return a + b + c - max({a, b, c}) - min({a, b, c});
}
void ans1(bool x) {
  if (x)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void ans2(bool x) {
  if (x)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void ans3(bool x) {
  if (x)
    cout << "Yay!" << endl;
  else
    cout << ":(" << endl;
}
template <typename T1, typename T2> void ans(bool x, T1 y, T2 z) {
  if (x)
    cout << y << endl;
  else
    cout << z << endl;
}
template <typename T> void debug(vector<vector<T>> &v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    cout << v[i][0];
    for (ll j = 1; j < w; j++)
      cout spa v[i][j];
    cout << endl;
  }
};
void debug(vector<string> &v, ll h, ll w) {
  for (ll i = 0; i < h; i++) {
    for (ll j = 0; j < w; j++)
      cout << v[i][j];
    cout << endl;
  }
};
template <typename T> void debug(vector<T> &v, ll n) {
  if (n != 0)
    cout << v[0];
  for (ll i = 1; i < n; i++)
    cout spa v[i];
  cout << endl;
};
template <typename T> vector<vector<T>> vec(ll x, ll y, T w) {
  vector<vector<T>> v(x, vector<T>(y, w));
  return v;
}
ll gcd(ll x, ll y) {
  ll r;
  while (y != 0 && (r = x % y) != 0) {
    x = y;
    y = r;
  }
  return y == 0 ? x : y;
}
vector<ll> dx = {1, -1, 0, 0, 1, 1, -1, -1};
vector<ll> dy = {0, 0, 1, -1, 1, -1, 1, -1};
template <typename T> vector<T> make_v(size_t a, T b) {
  return vector<T>(a, b);
}
template <typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  return os << p.first << " " << p.second;
}
// mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());

int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res = 0, buf = 0;
  bool judge = true;
  ll n;
  cin >> n;
  string s;
  cin >> s;
  vector<ll> a, b, c;
  rep(i, 0, 2 * n) {
    if (s[i] == 'a') {
      c.PB(a.size());
      a.PB(i);
    } else {
      c.PB(b.size());
      b.PB(i);
    }
  }
  auto f = [&](string t) {
    ll nxt = 0;
    queue<ll> ch;
    stack<ll> st;
    rep(i, 0, 2 * n) {
      if (nxt == t.size())
        break;
      if (!ch.empty() && ch.front() == i) {
        ch.pop();
        if (s[i] == t[nxt]) {
          st.push(c[i]);
          nxt++;
        } else {
          ll mi = min(a[c[i]], b[c[i]]);
          while (!st.empty()) {
            ll p = st.top();
            if (max(a[p], b[p]) > mi) {
              chmin(mi, min(a[p], b[p]));
              nxt -= 2;
              st.pop();
            } else
              break;
          }
          nxt--;
        }
      } else if (min(a[c[i]], b[c[i]]) == i && s[i] == t[nxt]) {
        nxt++;
        ch.push(max(a[c[i]], b[c[i]]));
      }
    }
    // cout<<t spa nxt<<endl;
    return nxt == t.size();
  };
  string ret;
  while (1) {
    if (f(ret + 'b')) {
      ret += 'b';
    } else if (f(ret + 'a')) {
      ret += 'a';
    } else
      break;
  }
  cout << ret << endl;
  return 0;
}