Problem p03355 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <random>
#include <regex>
#include <set>
#include <string>
#include <sys/timeb.h>
#include <vector>

using namespace std;

#define DEBUG_

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define reprrev(i, a, b) for (int i = (int)(b)-1; i >= (int)(a); i--)
#define reprev(i, n) reprrev(i, 0, n)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define mp make_pair
#define mt make_tuple
#define INF 1050000000
#define INFR INT_MAX
#define INFL (long long)(4e18)
#define INFLR LLONG_MAX
#define EPS (1e-10)
#define MOD 1000000007
#define PI 3.141592653589793238
#define RMAX 4294967295

#ifdef DEBUG_
#define DEB
#else
#define DEB if (false)
#endif

#ifdef DEBUG_
#define dump(...)                                                              \
  DUMPOUT << "  ";                                                             \
  dump_func(string(#__VA_ARGS__) + ":",                                        \
            "[" + to_string(__LINE__) + ":" + __FUNCTION__ + "]");             \
  DUMPOUT << "    ";                                                           \
  dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

typedef long long ll;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<Pi> vPi;
typedef vector<Pll> vPll;
typedef vector<Pd> vPd;
typedef vector<vector<int>> vvi;
typedef vector<vector<bool>> vvb;
typedef vector<vector<ll>> vvll;
typedef vector<vector<char>> vvc;
typedef vector<vector<string>> vvs;
typedef vector<vector<double>> vvd;
typedef vector<vector<Pi>> vvPi;
typedef vector<vector<vector<int>>> vvvi;
typedef vector<vector<vector<ll>>> vvvll;
typedef priority_queue<int, vector<int>, greater<int>> pqli;
typedef priority_queue<ll, vector<ll>, greater<ll>> pqlll;
typedef priority_queue<Pi, vector<Pi>, greater<Pi>> pqlP;
template <class T> using vec = vector<T>;
template <class T> using pql = priority_queue<T, vector<T>, greater<T>>;
string debug_show(Pi a) {
  return "(" + to_string(a.first) + "," + to_string(a.second) + ")";
}

#define DUMPOUT cerr

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  DUMPOUT << head;
  if (sizeof...(Tail) <= 1) {
    DUMPOUT << " ";
  } else {
    DUMPOUT << ", ";
  }
  dump_func(std::move(tail)...);
}

// vector
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
  os << "(" << pair_var.first << "," << pair_var.second << ")";
  return os;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &pair_var) {
  os << "(" << pair_var.first << "," << pair_var.second << ")";
  return os;
}
// vector
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
  }
  os << "}";
  return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << "(" << itr->first << "," << itr->second << ")";
    itr++;
    if (itr == map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
// set
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) {
  os << "{";
  repi(itr, set_var) {
    os << "  " << *itr << endl;
    itr++;
    if (itr == set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
string YN(bool y, int id = 0) {
  if (id)
    cout << id;
  return (y ? "YES" : "NO");
}
string yn(bool y, int id = 0) {
  if (id)
    cout << id;
  return (y ? "Yes" : "No");
}
string ON(bool y, int id = 0) {
  if (id)
    cout << id;
  return (y ? "OK" : "NG");
}

int dir4[4][2] = {{0, -1}, {-1, 0}, {1, 0}, {0, 1}};
int dir8[8][2] = {{-1, -1}, {0, -1}, {1, -1}, {-1, 0},
                  {1, 0},   {-1, 1}, {0, 1},  {1, 1}};

// [a,b)
int irand(int a, int b) {
  static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
  uniform_int_distribution<int> dist(a, b - 1);
  return dist(Rand);
}

// [a,b)
double drand(int a, int b) {
  static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
  uniform_real_distribution<double> dist(a, b);
  return dist(Rand);
}
struct Edge {
  int from, to, cost;
};
using Graph = vec<vec<Edge>>;

//======================================================

int main() {
  string s;
  int K;
  cin >> s >> K;
  pql<string> Q;
  vi num(26, 0);

  reprev(i, s.size()) { num[s[i] - 'a'] = s.size() - i; }

  int c = 0;
  int cnt = 0;
  rep(i, 26) {
    cnt += num[i];
    if (cnt >= K) {
      c = i;
      break;
    }
  }

  set<string> exist;

  rep(i, s.size()) {
    if (s[i] - 'a' > c)
      continue;
    repr(j, 1, s.size() - i + 1) {
      string t = s.substr(i, j);
      if (exist.find(t) == exist.end()) {
        exist.insert(t);
        Q.push(t);
      }
    }
  }

  rep(i, K - 1) { Q.pop(); }

  cout << Q.top() << endl;

  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <random>
#include <regex>
#include <set>
#include <string>
#include <sys/timeb.h>
#include <vector>

using namespace std;

#define DEBUG_

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define reprrev(i, a, b) for (int i = (int)(b)-1; i >= (int)(a); i--)
#define reprev(i, n) reprrev(i, 0, n)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define mp make_pair
#define mt make_tuple
#define INF 1050000000
#define INFR INT_MAX
#define INFL (long long)(4e18)
#define INFLR LLONG_MAX
#define EPS (1e-10)
#define MOD 1000000007
#define PI 3.141592653589793238
#define RMAX 4294967295

#ifdef DEBUG_
#define DEB
#else
#define DEB if (false)
#endif

#ifdef DEBUG_
#define dump(...)                                                              \
  DUMPOUT << "  ";                                                             \
  dump_func(string(#__VA_ARGS__) + ":",                                        \
            "[" + to_string(__LINE__) + ":" + __FUNCTION__ + "]");             \
  DUMPOUT << "    ";                                                           \
  dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

typedef long long ll;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<Pi> vPi;
typedef vector<Pll> vPll;
typedef vector<Pd> vPd;
typedef vector<vector<int>> vvi;
typedef vector<vector<bool>> vvb;
typedef vector<vector<ll>> vvll;
typedef vector<vector<char>> vvc;
typedef vector<vector<string>> vvs;
typedef vector<vector<double>> vvd;
typedef vector<vector<Pi>> vvPi;
typedef vector<vector<vector<int>>> vvvi;
typedef vector<vector<vector<ll>>> vvvll;
typedef priority_queue<int, vector<int>, greater<int>> pqli;
typedef priority_queue<ll, vector<ll>, greater<ll>> pqlll;
typedef priority_queue<Pi, vector<Pi>, greater<Pi>> pqlP;
template <class T> using vec = vector<T>;
template <class T> using pql = priority_queue<T, vector<T>, greater<T>>;
string debug_show(Pi a) {
  return "(" + to_string(a.first) + "," + to_string(a.second) + ")";
}

#define DUMPOUT cerr

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&...tail) {
  DUMPOUT << head;
  if (sizeof...(Tail) <= 1) {
    DUMPOUT << " ";
  } else {
    DUMPOUT << ", ";
  }
  dump_func(std::move(tail)...);
}

// vector
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
  os << "(" << pair_var.first << "," << pair_var.second << ")";
  return os;
}
// pair
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &pair_var) {
  os << "(" << pair_var.first << "," << pair_var.second << ")";
  return os;
}
// vector
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "{";
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
  }
  os << "}";
  return os;
}
// map
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
  os << "{";
  repi(itr, map_var) {
    os << "(" << itr->first << "," << itr->second << ")";
    itr++;
    if (itr == map_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
// set
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) {
  os << "{";
  repi(itr, set_var) {
    os << "  " << *itr << endl;
    itr++;
    if (itr == set_var.end())
      os << ", ";
    itr--;
  }
  os << "}";
  return os;
}
string YN(bool y, int id = 0) {
  if (id)
    cout << id;
  return (y ? "YES" : "NO");
}
string yn(bool y, int id = 0) {
  if (id)
    cout << id;
  return (y ? "Yes" : "No");
}
string ON(bool y, int id = 0) {
  if (id)
    cout << id;
  return (y ? "OK" : "NG");
}

int dir4[4][2] = {{0, -1}, {-1, 0}, {1, 0}, {0, 1}};
int dir8[8][2] = {{-1, -1}, {0, -1}, {1, -1}, {-1, 0},
                  {1, 0},   {-1, 1}, {0, 1},  {1, 1}};

// [a,b)
int irand(int a, int b) {
  static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
  uniform_int_distribution<int> dist(a, b - 1);
  return dist(Rand);
}

// [a,b)
double drand(int a, int b) {
  static mt19937 Rand(static_cast<unsigned int>(time(nullptr)));
  uniform_real_distribution<double> dist(a, b);
  return dist(Rand);
}
struct Edge {
  int from, to, cost;
};
using Graph = vec<vec<Edge>>;

//======================================================

int main() {
  string s;
  int K;
  cin >> s >> K;
  pql<string> Q;
  vi num(26, 0);

  reprev(i, s.size()) { num[s[i] - 'a'] = s.size() - i; }

  int c = 0;
  int cnt = 0;
  rep(i, 26) {
    cnt += num[i];
    if (cnt >= K) {
      c = i;
      break;
    }
  }

  set<string> exist;

  rep(i, s.size()) {
    if (s[i] - 'a' > c)
      continue;
    repr(j, 1, min(K + 1, (int)s.size() - i + 1)) {
      string t = s.substr(i, j);
      if (exist.find(t) == exist.end()) {
        exist.insert(t);
        Q.push(t);
      }
    }
  }

  rep(i, K - 1) { Q.pop(); }

  cout << Q.top() << endl;

  return 0;
}