Problem p03353 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
using ll = long long;
using ull = unsigned long long;
using uint = unsigned int;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;
const ll LINF = 0x3fffffffffffffff;
const ll MOD = 1000000007;
const ll MODD = 0x3b800001;
const int INF = 0x3fffffff;
const double DINF = numeric_limits<double>::infinity();
const vector<pii> four = {{-1, 0}, {0, 1}, {1, 0}, {0, -1}};
#define _overload4(_1, _2, _3, _4, name, ...) name
#define _overload3(_1, _2, _3, name, ...) name
#define _rep1(n) _rep2(i, n)
#define _rep2(i, n) _rep3(i, 0, n)
#define _rep3(i, a, b) for (ll i = a; i < b; ++i)
#define _rep4(i, a, b, c) for (ll i = a; i < b; i += c)
#define rep(...)                                                               \
  _overload4(__VA_ARGS__, _rep4, _rep3, _rep2, _rep1)(__VA_ARGS__)
#define _rrep1(n) _rrep2(i, n)
#define _rrep2(i, n) _rrep3(i, 0, n)
#define _rrep3(i, a, b) for (ll i = b - 1; i >= a; i--)
#define _rrep4(i, a, b, c) for (ll i = a + (b - a - 1) / c * c; i >= a; i -= c)
#define rrep(...)                                                              \
  _overload4(__VA_ARGS__, _rrep4, _rrep3, _rrep2, _rrep1)(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define sum(...) accumulate(range(__VA_ARGS__), 0LL)
#define dsum(...) accumulate(range(__VA_ARGS__), double(0))
#define _range(i) (i).begin(), (i).end()
#define _range2(i, k) (i).begin(), (i).begin() + k
#define _range3(i, a, b) (i).begin() + a, (i).begin() + b
#define range(...)                                                             \
  _overload3(__VA_ARGS__, _range3, _range2, _range)(__VA_ARGS__)
#define yes(i) out(i ? "yes" : "no")
#define Yes(i) out(i ? "Yes" : "No")
#define YES(i) out(i ? "YES" : "NO")
#define Possible(i) out(i ? "Possible" : "Impossible")
#define elif else if
#define unless(a) if (!(a))
#define mp make_pair
#define INT(...)                                                               \
  int __VA_ARGS__;                                                             \
  in(__VA_ARGS__)
#define LL(...)                                                                \
  ll __VA_ARGS__;                                                              \
  in(__VA_ARGS__)
#define STR(...)                                                               \
  string __VA_ARGS__;                                                          \
  in(__VA_ARGS__)
#define CHR(...)                                                               \
  char __VA_ARGS__;                                                            \
  in(__VA_ARGS__)
#define DBL(...)                                                               \
  double __VA_ARGS__;                                                          \
  in(__VA_ARGS__)
#define vec(type, name, ...) vector<type> name(__VA_ARGS__)
#define VEC(type, name, size)                                                  \
  vector<type> name(size);                                                     \
  in(name)
#define vv(type, name, h, ...)                                                 \
  vector<vector<type>> name(h, vector<type>(__VA_ARGS__))
#define VV(type, name, h, ...)                                                 \
  vector<vector<type>> name(h, vector<type>(__VA_ARGS__));                     \
  in(name)
__attribute__((constructor)) void SETTINGS() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(15);
};
template <class T> inline constexpr T gcd(T a, T b) {
  if (!a || !b)
    return 0;
  return a % b ? gcd(b, a % b) : b;
}
template <class T> inline constexpr T lcm(T a, T b) { a *b / gcd(a, b); }
template <class T> inline constexpr T min(vector<T> &v) {
  return *min_element(range(v));
}
inline char min(string &v) { return *min_element(range(v)); }
template <class T> inline constexpr T max(vector<T> &v) {
  return *max_element(range(v));
}
inline char max(string &v) { return *max_element(range(v)); }
template <typename T> inline bool update_min(T &mn, const T &cnt) {
  if (mn > cnt) {
    mn = cnt;
    return 1;
  } else
    return 0;
}
template <typename T> inline bool update_max(T &mx, const T &cnt) {
  if (mx < cnt) {
    mx = cnt;
    return 1;
  } else
    return 0;
}
inline void in() {}
template <class T> istream &operator>>(istream &is, vector<T> &vec);
template <class T, size_t size>
istream &operator>>(istream &is, array<T, size> &vec);
template <class T, class L> istream &operator>>(istream &is, pair<T, L> &p);
template <class T> ostream &operator<<(ostream &os, vector<T> &vec);
template <class T, class L> ostream &operator<<(ostream &os, pair<T, L> &p);
template <class T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
template <class T, class L> istream &operator>>(istream &is, pair<T, L> &p) {
  is >> p.first;
  is >> p.second;
  return is;
}
template <class T> ostream &operator<<(ostream &os, vector<T> &vec) {
  os << vec[0];
  rep(i, 1, vec.size()) { os << ' ' << vec[i]; }
  return os;
}
template <class T> ostream &operator<<(ostream &os, deque<T> &deq) {
  os << deq[0];
  rep(i, 1, deq.size()) { os << ' ' << deq[i]; }
  return os;
}
template <class T, class L> ostream &operator<<(ostream &os, pair<T, L> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <class T, class L> pair<T, L> operator+(pair<T, L> a, pair<T, L> b) {
  return {a.first + b.first, a.second + b.second};
}
template <class Head, class... Tail>
inline void in(Head &&head, Tail &&...tail) {
  cin >> head;
  in(move(tail)...);
}
template <class T> inline bool out(T t) {
  cout << t << '\n';
  return 0;
}
inline bool out() {
  cout << '\n';
  return 0;
}
template <class Head, class... Tail> inline bool out(Head head, Tail... tail) {
  cout << head << ' ';
  out(move(tail)...);
  return 0;
}
template <class T> inline void err(T t) { cerr << t << '\n'; }
inline void err() { cerr << '\n'; }
template <class Head, class... Tail> inline void err(Head head, Tail... tail) {
  cerr << head << ' ';
  out(move(tail)...);
}

signed main() {
  STR(s);
  LL(k);
  set<string, greater<string>> q;
  rep(i, 0, s.size()) rep(j, 1, s.size() - i + 1) {
    q.insert(s.substr(i, j));
    if (q.size() > k)
      q.erase(q.begin());
  }
  out(*q.begin());
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
using ll = long long;
using ull = unsigned long long;
using uint = unsigned int;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;
const ll LINF = 0x3fffffffffffffff;
const ll MOD = 1000000007;
const ll MODD = 0x3b800001;
const int INF = 0x3fffffff;
const double DINF = numeric_limits<double>::infinity();
const vector<pii> four = {{-1, 0}, {0, 1}, {1, 0}, {0, -1}};
#define _overload4(_1, _2, _3, _4, name, ...) name
#define _overload3(_1, _2, _3, name, ...) name
#define _rep1(n) _rep2(i, n)
#define _rep2(i, n) _rep3(i, 0, n)
#define _rep3(i, a, b) for (ll i = a; i < b; ++i)
#define _rep4(i, a, b, c) for (ll i = a; i < b; i += c)
#define rep(...)                                                               \
  _overload4(__VA_ARGS__, _rep4, _rep3, _rep2, _rep1)(__VA_ARGS__)
#define _rrep1(n) _rrep2(i, n)
#define _rrep2(i, n) _rrep3(i, 0, n)
#define _rrep3(i, a, b) for (ll i = b - 1; i >= a; i--)
#define _rrep4(i, a, b, c) for (ll i = a + (b - a - 1) / c * c; i >= a; i -= c)
#define rrep(...)                                                              \
  _overload4(__VA_ARGS__, _rrep4, _rrep3, _rrep2, _rrep1)(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define sum(...) accumulate(range(__VA_ARGS__), 0LL)
#define dsum(...) accumulate(range(__VA_ARGS__), double(0))
#define _range(i) (i).begin(), (i).end()
#define _range2(i, k) (i).begin(), (i).begin() + k
#define _range3(i, a, b) (i).begin() + a, (i).begin() + b
#define range(...)                                                             \
  _overload3(__VA_ARGS__, _range3, _range2, _range)(__VA_ARGS__)
#define yes(i) out(i ? "yes" : "no")
#define Yes(i) out(i ? "Yes" : "No")
#define YES(i) out(i ? "YES" : "NO")
#define Possible(i) out(i ? "Possible" : "Impossible")
#define elif else if
#define unless(a) if (!(a))
#define mp make_pair
#define INT(...)                                                               \
  int __VA_ARGS__;                                                             \
  in(__VA_ARGS__)
#define LL(...)                                                                \
  ll __VA_ARGS__;                                                              \
  in(__VA_ARGS__)
#define STR(...)                                                               \
  string __VA_ARGS__;                                                          \
  in(__VA_ARGS__)
#define CHR(...)                                                               \
  char __VA_ARGS__;                                                            \
  in(__VA_ARGS__)
#define DBL(...)                                                               \
  double __VA_ARGS__;                                                          \
  in(__VA_ARGS__)
#define vec(type, name, ...) vector<type> name(__VA_ARGS__)
#define VEC(type, name, size)                                                  \
  vector<type> name(size);                                                     \
  in(name)
#define vv(type, name, h, ...)                                                 \
  vector<vector<type>> name(h, vector<type>(__VA_ARGS__))
#define VV(type, name, h, ...)                                                 \
  vector<vector<type>> name(h, vector<type>(__VA_ARGS__));                     \
  in(name)
__attribute__((constructor)) void SETTINGS() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(15);
};
template <class T> inline constexpr T gcd(T a, T b) {
  if (!a || !b)
    return 0;
  return a % b ? gcd(b, a % b) : b;
}
template <class T> inline constexpr T lcm(T a, T b) { a *b / gcd(a, b); }
template <class T> inline constexpr T min(vector<T> &v) {
  return *min_element(range(v));
}
inline char min(string &v) { return *min_element(range(v)); }
template <class T> inline constexpr T max(vector<T> &v) {
  return *max_element(range(v));
}
inline char max(string &v) { return *max_element(range(v)); }
template <typename T> inline bool update_min(T &mn, const T &cnt) {
  if (mn > cnt) {
    mn = cnt;
    return 1;
  } else
    return 0;
}
template <typename T> inline bool update_max(T &mx, const T &cnt) {
  if (mx < cnt) {
    mx = cnt;
    return 1;
  } else
    return 0;
}
inline void in() {}
template <class T> istream &operator>>(istream &is, vector<T> &vec);
template <class T, size_t size>
istream &operator>>(istream &is, array<T, size> &vec);
template <class T, class L> istream &operator>>(istream &is, pair<T, L> &p);
template <class T> ostream &operator<<(ostream &os, vector<T> &vec);
template <class T, class L> ostream &operator<<(ostream &os, pair<T, L> &p);
template <class T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
template <class T, class L> istream &operator>>(istream &is, pair<T, L> &p) {
  is >> p.first;
  is >> p.second;
  return is;
}
template <class T> ostream &operator<<(ostream &os, vector<T> &vec) {
  os << vec[0];
  rep(i, 1, vec.size()) { os << ' ' << vec[i]; }
  return os;
}
template <class T> ostream &operator<<(ostream &os, deque<T> &deq) {
  os << deq[0];
  rep(i, 1, deq.size()) { os << ' ' << deq[i]; }
  return os;
}
template <class T, class L> ostream &operator<<(ostream &os, pair<T, L> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <class T, class L> pair<T, L> operator+(pair<T, L> a, pair<T, L> b) {
  return {a.first + b.first, a.second + b.second};
}
template <class Head, class... Tail>
inline void in(Head &&head, Tail &&...tail) {
  cin >> head;
  in(move(tail)...);
}
template <class T> inline bool out(T t) {
  cout << t << '\n';
  return 0;
}
inline bool out() {
  cout << '\n';
  return 0;
}
template <class Head, class... Tail> inline bool out(Head head, Tail... tail) {
  cout << head << ' ';
  out(move(tail)...);
  return 0;
}
template <class T> inline void err(T t) { cerr << t << '\n'; }
inline void err() { cerr << '\n'; }
template <class Head, class... Tail> inline void err(Head head, Tail... tail) {
  cerr << head << ' ';
  out(move(tail)...);
}

signed main() {
  STR(s);
  LL(k);
  set<string, greater<string>> q;
  rep(i, 0, s.size()) rep(j, 1, min(ll(s.size()) - i, k) + 1) {
    q.insert(s.substr(i, j));
    if (q.size() > k)
      q.erase(q.begin());
  }
  out(*q.begin());
}