Problem p03353 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <stdlib.h>

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
using default_counter_t = int;

// macro
#define let auto const &
#define overload4(a, b, c, d, name, ...) name
#define rep1(n)                                                                \
  for (default_counter_t i = 0, end_i = default_counter_t(n); i < end_i; ++i)
#define rep2(i, n)                                                             \
  for (default_counter_t i = 0, end_##i = default_counter_t(n); i < end_##i;   \
       ++i)
#define rep3(i, a, b)                                                          \
  for (default_counter_t i = default_counter_t(a),                             \
                         end_##i = default_counter_t(b);                       \
       i < end_##i; ++i)
#define rep4(i, a, b, c)                                                       \
  for (default_counter_t i = default_counter_t(a),                             \
                         end_##i = default_counter_t(b);                       \
       i < end_##i; i += default_counter_t(c))
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)

#define rrep1(n)                                                               \
  for (default_counter_t i = default_counter_t(n) - 1; i >= 0; --i)
#define rrep2(i, n)                                                            \
  for (default_counter_t i = default_counter_t(n) - 1; i >= 0; --i)
#define rrep3(i, a, b)                                                         \
  for (default_counter_t i = default_counter_t(b) - 1,                         \
                         begin_##i = default_counter_t(a);                     \
       i >= begin_##i; --i)
#define rrep4(i, a, b, c)                                                      \
  for (int i = (default_counter_t(b) - default_counter_t(a) - 1) /             \
                   default_counter_t(c) * default_counter_t(c) +               \
               default_counter_t(a),                                           \
           begin_##i = default_counter_t(a);                                   \
       i >= begin_##i; i -= default_counter_t(c))
#define rrep(...)                                                              \
  overload4(__VA_ARGS__, rrep4, rrep3, rrep2, rrep1)(__VA_ARGS__)

#define ALL(f, c, ...)                                                         \
  (([&](decltype((c)) cccc) {                                                  \
    return (f)(std::begin(cccc), std::end(cccc), ##__VA_ARGS__);               \
  })(c))

// function
template <class C> constexpr C &Sort(C &a) {
  std::sort(std::begin(a), std::end(a));
  return a;
}
template <class C> constexpr auto &Min(C const &a) {
  return *std::min_element(std::begin(a), std::end(a));
}
template <class C> constexpr auto &Max(C const &a) {
  return *std::max_element(std::begin(a), std::end(a));
}
template <class C> constexpr auto Total(C const &c) {
  return std::accumulate(std::begin(c), std::end(c), C(0));
}
template <typename T> auto CumSum(std::vector<T> const &v) {
  std::vector<T> a(v.size() + 1, T(0));
  for (std::size_t i = 0; i < a.size() - size_t(1); ++i)
    a[i + 1] = a[i] + v[i];
  return a;
}
template <typename T> constexpr bool ChMax(T &a, T const &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> constexpr bool ChMin(T &a, T const &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

void In(void) { return; }

template <typename First, typename... Rest>
void In(First &first, Rest &...rest) {
  cin >> first;
  In(rest...);
  return;
}

template <class T, typename I> void VectorIn(vector<T> &v, const I n) {
  v.resize(size_t(n));
  rep(i, v.size()) cin >> v[i];
}

void Out(void) {
  cout << "\n";
  return;
}

template <typename First, typename... Rest>
void Out(First first, Rest... rest) {
  cout << first << " ";
  Out(rest...);
  return;
}

constexpr auto yes(const bool c) { return c ? "yes" : "no"; }
constexpr auto Yes(const bool c) { return c ? "Yes" : "No"; }
constexpr auto YES(const bool c) { return c ? "YES" : "NO"; }

#ifdef USE_STACK_TRACE_LOGGER
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Weverything"
#include <glog/logging.h>
#pragma clang diagnostic pop
#endif //__clang__
#endif // USE_STACK_TRACE_LOGGER

#include <set>

signed main(int argc, char *argv[]) {
  (void)argc;
#ifdef USE_STACK_TRACE_LOGGER
  google::InitGoogleLogging(argv[0]);
  google::InstallFailureSignalHandler();
#else
  (void)argv;
#endif // USE_STACK_TRACE_LOGGER
  string s;
  int64_t K;
  In(s, K);

  let n = s.size();
  set<string> ss;
  rep(c, 'a', 'z' + 1) {
    rep(i, n) {
      if (c == s[i]) {
        rep(j, 1, n - i + 1) { ss.insert(s.substr(i, j)); }
      }
    }
    if (int64_t(ss.size()) >= K)
      break;
  }
  auto it = ss.begin();
  rep(i, K - 1) it++;
  Out(*it);
  return EXIT_SUCCESS;
}


Fixed submission:
#include <stdlib.h>

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
using default_counter_t = int;

// macro
#define let auto const &
#define overload4(a, b, c, d, name, ...) name
#define rep1(n)                                                                \
  for (default_counter_t i = 0, end_i = default_counter_t(n); i < end_i; ++i)
#define rep2(i, n)                                                             \
  for (default_counter_t i = 0, end_##i = default_counter_t(n); i < end_##i;   \
       ++i)
#define rep3(i, a, b)                                                          \
  for (default_counter_t i = default_counter_t(a),                             \
                         end_##i = default_counter_t(b);                       \
       i < end_##i; ++i)
#define rep4(i, a, b, c)                                                       \
  for (default_counter_t i = default_counter_t(a),                             \
                         end_##i = default_counter_t(b);                       \
       i < end_##i; i += default_counter_t(c))
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)

#define rrep1(n)                                                               \
  for (default_counter_t i = default_counter_t(n) - 1; i >= 0; --i)
#define rrep2(i, n)                                                            \
  for (default_counter_t i = default_counter_t(n) - 1; i >= 0; --i)
#define rrep3(i, a, b)                                                         \
  for (default_counter_t i = default_counter_t(b) - 1,                         \
                         begin_##i = default_counter_t(a);                     \
       i >= begin_##i; --i)
#define rrep4(i, a, b, c)                                                      \
  for (int i = (default_counter_t(b) - default_counter_t(a) - 1) /             \
                   default_counter_t(c) * default_counter_t(c) +               \
               default_counter_t(a),                                           \
           begin_##i = default_counter_t(a);                                   \
       i >= begin_##i; i -= default_counter_t(c))
#define rrep(...)                                                              \
  overload4(__VA_ARGS__, rrep4, rrep3, rrep2, rrep1)(__VA_ARGS__)

#define ALL(f, c, ...)                                                         \
  (([&](decltype((c)) cccc) {                                                  \
    return (f)(std::begin(cccc), std::end(cccc), ##__VA_ARGS__);               \
  })(c))

// function
template <class C> constexpr C &Sort(C &a) {
  std::sort(std::begin(a), std::end(a));
  return a;
}
template <class C> constexpr auto &Min(C const &a) {
  return *std::min_element(std::begin(a), std::end(a));
}
template <class C> constexpr auto &Max(C const &a) {
  return *std::max_element(std::begin(a), std::end(a));
}
template <class C> constexpr auto Total(C const &c) {
  return std::accumulate(std::begin(c), std::end(c), C(0));
}
template <typename T> auto CumSum(std::vector<T> const &v) {
  std::vector<T> a(v.size() + 1, T(0));
  for (std::size_t i = 0; i < a.size() - size_t(1); ++i)
    a[i + 1] = a[i] + v[i];
  return a;
}
template <typename T> constexpr bool ChMax(T &a, T const &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> constexpr bool ChMin(T &a, T const &b) {
  if (b < a) {
    a = b;
    return true;
  }
  return false;
}

void In(void) { return; }

template <typename First, typename... Rest>
void In(First &first, Rest &...rest) {
  cin >> first;
  In(rest...);
  return;
}

template <class T, typename I> void VectorIn(vector<T> &v, const I n) {
  v.resize(size_t(n));
  rep(i, v.size()) cin >> v[i];
}

void Out(void) {
  cout << "\n";
  return;
}

template <typename First, typename... Rest>
void Out(First first, Rest... rest) {
  cout << first << " ";
  Out(rest...);
  return;
}

constexpr auto yes(const bool c) { return c ? "yes" : "no"; }
constexpr auto Yes(const bool c) { return c ? "Yes" : "No"; }
constexpr auto YES(const bool c) { return c ? "YES" : "NO"; }

#ifdef USE_STACK_TRACE_LOGGER
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Weverything"
#include <glog/logging.h>
#pragma clang diagnostic pop
#endif //__clang__
#endif // USE_STACK_TRACE_LOGGER

#include <set>

signed main(int argc, char *argv[]) {
  (void)argc;
#ifdef USE_STACK_TRACE_LOGGER
  google::InitGoogleLogging(argv[0]);
  google::InstallFailureSignalHandler();
#else
  (void)argv;
#endif // USE_STACK_TRACE_LOGGER
  string s;
  int64_t K;
  In(s, K);

  let n = s.size();
  set<string> ss;
  rep(c, 'a', 'z' + 1) {
    rep(i, n) {
      if (c == s[i]) {
        rep(j, 1, min<int64_t>(K + 1, n - i + 1)) { ss.insert(s.substr(i, j)); }
      }
    }
    if (int64_t(ss.size()) >= K)
      break;
  }
  auto it = ss.begin();
  rep(i, K - 1) it++;
  Out(*it);
  return EXIT_SUCCESS;
}