Problem p03355 (C++)

Buggy submission (status: Time Limit Exceeded):
// May this submission get accepted
#include <bits/stdc++.h>

// エイリアス
using ll = long signed long;
using ull = long unsigned long;
using ld = long double;
using namespace std;

// エイリアス (補完・コンパイルが重くなる)
// #include <boost/multiprecision/cpp_int.hpp>
// using mll = boost::multiprecision::cpp_int;

// 汎用マクロ
#define ALL_OF(x) (x).begin(), (x).end()
#define REP(i, n) for (long long i = 0, i##_len = (n); i < i##_len; i++)
#define RANGE(i, is, ie)                                                       \
  for (long long i = (is), i##_end = (ie); i <= i##_end; i++)
#define DSRNG(i, is, ie)                                                       \
  for (long long i = (is), i##_end = (ie); i >= i##_end; i--)
#define STEP(i, is, ie, step)                                                  \
  for (long long i = (is), i##_end = (ie), i##_step = (step); i <= i##_end;    \
       i += i##_step)
#define UNIQUE(v)                                                              \
  do {                                                                         \
    sort((v).begin(), (v).end());                                              \
    (v).erase(unique((v).begin(), (v).end()), (v).end());                      \
  } while (false)
#define FOREACH(i, q) for (auto &i : q)
template <class T> bool chmax(T &a, const T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
constexpr int INF = numeric_limits<int>::max();
constexpr long long LINF = numeric_limits<long long>::max();
constexpr long double EPS = 1e-10L;
#define Yes(q) ((q) ? "Yes" : "No")
#define YES(q) ((q) ? "YES" : "NO")
#define Possible(q) ((q) ? "Possible" : "Impossible")
#define POSSIBLE(q) ((q) ? "POSSIBLE" : "IMPOSSIBLE")
#define IIF(q, t, f) ((q) ? (t) : (f))
#define DUMP(q) DUMP_FUNC(q, #q, __FILE__, __LINE__)
template <typename T> void DUMP_PROC(T x) {
  if (is_integral<T>() || is_floating_point<T>())
    cerr << "\e[32m" << x << "\e[m";
  else
    cerr << x;
}
template <> void DUMP_PROC<char>(char x) {
  cerr << "\e[36m\'" << x << "\'\e[m";
}
template <> void DUMP_PROC<string>(string x) {
  cerr << "\e[33m\"" << x << "\"\e[m";
}
template <typename T, typename U> void DUMP_PROC(pair<T, U> x) {
  cerr << "{";
  DUMP_PROC(x.first);
  cerr << ", ";
  DUMP_PROC(x.second);
  cerr << "}";
}
template <typename... T, typename U, U... Seq>
void DUMP_PROC(tuple<T...> &x, integer_sequence<U, Seq...>) {
  (void)(int[]){(cerr << ((const char *[]){"", ", "})[!!Seq]
                      << (DUMP_PROC(get<Seq>(x)), ""),
                 0)...};
}
template <typename... T> void DUMP_PROC(tuple<T...> x) {
  cerr << "{";
  DUMP_PROC(x, index_sequence_for<T...>());
  cerr << "}";
}
template <typename T> void DUMP_PROC(vector<T> x) {
  cerr << "[";
  for (auto &xi : x) {
    DUMP_PROC(xi);
    cerr << (&xi != &*x.rbegin() ? ", " : "");
  }
  cerr << "]";
}
template <typename T>
void DUMP_FUNC(T x, const char *name, const char *fn, int ln) {
  cerr << "\e[32m[DEBUG]\e[m " << name << ": ";
  DUMP_PROC(x);
  cerr << " @ " << fn << "(" << ln << ")" << endl;
}
template <class T> T gcd(T a, T b) {
  if (a < b)
    swap(a, b);
  while (b)
    swap(a %= b, b);
  return a;
}
template <class T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

// gcc拡張マクロ
#define popcount __builtin_popcount
#define popcountll __builtin_popcountll

// 標準入出力
struct qin { // query input
  size_t sz;
  qin(size_t _sz = 1) : sz(_sz) {}
  template <typename T> operator T() const {
    T a;
    cin >> a;
    return a;
  }
  template <typename T> operator vector<T>() const {
    vector<T> a(sz);
    for (size_t i = 0; i < sz; i++)
      cin >> a[i];
    return a;
  }
  template <typename T, typename U> operator pair<T, U>() const {
    T f;
    U s;
    cin >> f >> s;
    return pair<T, U>(f, s);
  }
};
qin in1; // input one
template <typename T> void say(const T x, const char *end = "\n") {
  cout << x << end;
}
void say(const ld x, const char *end = "\n") {
  cout << setprecision(30) << x << end;
}
template <typename T>
void say(const vector<T> x, const char *sep = " ", const char *end = "\n") {
  REP(i, x.size()) { cout << x[i] << (i + 1 == i_len ? end : sep); }
}
template <typename T>
void say(const vector<vector<T>> x, const char *sep = " ",
         const char *end = "\n") {
  REP(i, x.size()) { say(x[i], sep, end); }
}

// モジュール
// [[LIBRARY]]

// 処理内容
int main() {

  ios::sync_with_stdio(false); // stdioを使うときはコメントアウトすること
  cin.tie(nullptr); // インタラクティブ問題ではコメントアウトすること

  string s = in1;
  ll k = in1;

  set<string> ss;
  for (char c = 'a'; c <= 'z'; c++) {
    REP(i, s.size()) {
      if (s[i] == c) {
        RANGE(j, i + 1, s.size()) {
          string t(s.begin() + i, s.begin() + j);
          ss.insert(t);
        }
      }
    }
    if (ss.size() >= k)
      break;
  }
  vector<string> sss;
  sss.reserve(ss.size());
  for (const string &x : ss)
    sss.push_back(x);
  sort(ALL_OF(sss));
  say(sss[k - 1]);
}

Fixed submission:
// May this submission get accepted
#include <bits/stdc++.h>

// エイリアス
using ll = long signed long;
using ull = long unsigned long;
using ld = long double;
using namespace std;

// エイリアス (補完・コンパイルが重くなる)
// #include <boost/multiprecision/cpp_int.hpp>
// using mll = boost::multiprecision::cpp_int;

// 汎用マクロ
#define ALL_OF(x) (x).begin(), (x).end()
#define REP(i, n) for (long long i = 0, i##_len = (n); i < i##_len; i++)
#define RANGE(i, is, ie)                                                       \
  for (long long i = (is), i##_end = (ie); i <= i##_end; i++)
#define DSRNG(i, is, ie)                                                       \
  for (long long i = (is), i##_end = (ie); i >= i##_end; i--)
#define STEP(i, is, ie, step)                                                  \
  for (long long i = (is), i##_end = (ie), i##_step = (step); i <= i##_end;    \
       i += i##_step)
#define UNIQUE(v)                                                              \
  do {                                                                         \
    sort((v).begin(), (v).end());                                              \
    (v).erase(unique((v).begin(), (v).end()), (v).end());                      \
  } while (false)
#define FOREACH(i, q) for (auto &i : q)
template <class T> bool chmax(T &a, const T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, const T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
constexpr int INF = numeric_limits<int>::max();
constexpr long long LINF = numeric_limits<long long>::max();
constexpr long double EPS = 1e-10L;
#define Yes(q) ((q) ? "Yes" : "No")
#define YES(q) ((q) ? "YES" : "NO")
#define Possible(q) ((q) ? "Possible" : "Impossible")
#define POSSIBLE(q) ((q) ? "POSSIBLE" : "IMPOSSIBLE")
#define IIF(q, t, f) ((q) ? (t) : (f))
#define DUMP(q) DUMP_FUNC(q, #q, __FILE__, __LINE__)
template <typename T> void DUMP_PROC(T x) {
  if (is_integral<T>() || is_floating_point<T>())
    cerr << "\e[32m" << x << "\e[m";
  else
    cerr << x;
}
template <> void DUMP_PROC<char>(char x) {
  cerr << "\e[36m\'" << x << "\'\e[m";
}
template <> void DUMP_PROC<string>(string x) {
  cerr << "\e[33m\"" << x << "\"\e[m";
}
template <typename T, typename U> void DUMP_PROC(pair<T, U> x) {
  cerr << "{";
  DUMP_PROC(x.first);
  cerr << ", ";
  DUMP_PROC(x.second);
  cerr << "}";
}
template <typename... T, typename U, U... Seq>
void DUMP_PROC(tuple<T...> &x, integer_sequence<U, Seq...>) {
  (void)(int[]){(cerr << ((const char *[]){"", ", "})[!!Seq]
                      << (DUMP_PROC(get<Seq>(x)), ""),
                 0)...};
}
template <typename... T> void DUMP_PROC(tuple<T...> x) {
  cerr << "{";
  DUMP_PROC(x, index_sequence_for<T...>());
  cerr << "}";
}
template <typename T> void DUMP_PROC(vector<T> x) {
  cerr << "[";
  for (auto &xi : x) {
    DUMP_PROC(xi);
    cerr << (&xi != &*x.rbegin() ? ", " : "");
  }
  cerr << "]";
}
template <typename T>
void DUMP_FUNC(T x, const char *name, const char *fn, int ln) {
  cerr << "\e[32m[DEBUG]\e[m " << name << ": ";
  DUMP_PROC(x);
  cerr << " @ " << fn << "(" << ln << ")" << endl;
}
template <class T> T gcd(T a, T b) {
  if (a < b)
    swap(a, b);
  while (b)
    swap(a %= b, b);
  return a;
}
template <class T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

// gcc拡張マクロ
#define popcount __builtin_popcount
#define popcountll __builtin_popcountll

// 標準入出力
struct qin { // query input
  size_t sz;
  qin(size_t _sz = 1) : sz(_sz) {}
  template <typename T> operator T() const {
    T a;
    cin >> a;
    return a;
  }
  template <typename T> operator vector<T>() const {
    vector<T> a(sz);
    for (size_t i = 0; i < sz; i++)
      cin >> a[i];
    return a;
  }
  template <typename T, typename U> operator pair<T, U>() const {
    T f;
    U s;
    cin >> f >> s;
    return pair<T, U>(f, s);
  }
};
qin in1; // input one
template <typename T> void say(const T x, const char *end = "\n") {
  cout << x << end;
}
void say(const ld x, const char *end = "\n") {
  cout << setprecision(30) << x << end;
}
template <typename T>
void say(const vector<T> x, const char *sep = " ", const char *end = "\n") {
  REP(i, x.size()) { cout << x[i] << (i + 1 == i_len ? end : sep); }
}
template <typename T>
void say(const vector<vector<T>> x, const char *sep = " ",
         const char *end = "\n") {
  REP(i, x.size()) { say(x[i], sep, end); }
}

// モジュール
// [[LIBRARY]]

// 処理内容
int main() {

  ios::sync_with_stdio(false); // stdioを使うときはコメントアウトすること
  cin.tie(nullptr); // インタラクティブ問題ではコメントアウトすること

  string s = in1;
  ll k = in1;

  set<string> ss;
  for (char c = 'a'; c <= 'z'; c++) {
    REP(i, s.size()) {
      if (s[i] == c) {
        RANGE(j, i + 1, min(i + k, (ll)s.size())) {
          string t(s.begin() + i, s.begin() + j);
          ss.insert(t);
        }
      }
    }
    if (ss.size() >= k)
      break;
  }
  vector<string> sss;
  sss.reserve(ss.size());
  for (const string &x : ss)
    sss.push_back(x);
  sort(ALL_OF(sss));
  say(sss[k - 1]);
}