Problem p03355 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;

using ll = long long;

#define REP(i, n) for (int i = 0; i < n; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define pb push_back
#define mp make_pair
#define fill(x, y) memset(x, y, sizeof(x))
#define even(x) x % 2 == 0
#define odd(x) x % 2 != 0
#define all(x) x.begin(), x.end()
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define F first
#define S second
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

// these functions return the position of result of Binary Search.
#define LB(s, t, x) (int)(lower_bound(s, t, x) - s)
#define UB(s, t, x) (int)(upper_bound(s, t, x) - s)

ll qp(ll a, ll b, int mo) {
  ll ans = 1;
  do {
    if (b & 1)
      ans = 1ll * ans * a % mo;
    a = 1ll * a * a % mo;
  } while (b >>= 1);
  return ans;
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts>
auto make_v(size_t a, size_t b, Ts... ts) {
  return vector<decltype(make_v<T>(b, ts...))>(a, make_v<T>(b, ts...));
}
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}
// auto dp = make_v<int>(4, h, w);
// fill_v(dp, 0);

const ll INF_LL = (1ll << 60);
const int INF_INT = (int)1e9;
const ll MOD_CONST = (ll)(1e9 + 7);

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

inline tuple<ll, ll> rotate45(tuple<ll, ll> p) {
  ll x = get<0>(p), y = get<1>(p);
  return tuple<ll, ll>(x + y, x - y);
}

bool comp(const std::string &lh, const std::string &rh) {
  // 文字列lhの長さが文字列rhより短かったらtrueを返す
  if (lh.length() == rh.length())
    return lh > rh;
  else
    return lh.length() > rh.length();
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(20);

  string s;
  cin >> s;
  ll N;
  cin >> N;

  char now = 'z';
  REP(i, s.size()) { chmin(now, s[i]); }
  set<string> ans;
  while (true) {
    REP(i, s.size()) {
      if (s[i] == now) {
        ans.insert(s.substr(i, 1));
        int cnt = 0;
        int num = max((int)s.size(), i + 4);
        FOR(j, i + 1, num) {
          ans.insert(s.substr(i, 2 + cnt));
          cnt++;
        }
      }
    }

    now++;
    if (ans.size() >= N)
      break;
  }

  auto itr = ans.begin();
  int cnt = 1;
  while (cnt < N)
    itr++, cnt++;
  cout << *itr << endl;

  return 0;
}


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

using ll = long long;

#define REP(i, n) for (int i = 0; i < n; i++)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define pb push_back
#define mp make_pair
#define fill(x, y) memset(x, y, sizeof(x))
#define even(x) x % 2 == 0
#define odd(x) x % 2 != 0
#define all(x) x.begin(), x.end()
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define F first
#define S second
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());

// these functions return the position of result of Binary Search.
#define LB(s, t, x) (int)(lower_bound(s, t, x) - s)
#define UB(s, t, x) (int)(upper_bound(s, t, x) - s)

ll qp(ll a, ll b, int mo) {
  ll ans = 1;
  do {
    if (b & 1)
      ans = 1ll * ans * a % mo;
    a = 1ll * a * a % mo;
  } while (b >>= 1);
  return ans;
}
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts>
auto make_v(size_t a, size_t b, Ts... ts) {
  return vector<decltype(make_v<T>(b, ts...))>(a, make_v<T>(b, ts...));
}
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}
// auto dp = make_v<int>(4, h, w);
// fill_v(dp, 0);

const ll INF_LL = (1ll << 60);
const int INF_INT = (int)1e9;
const ll MOD_CONST = (ll)(1e9 + 7);

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

inline tuple<ll, ll> rotate45(tuple<ll, ll> p) {
  ll x = get<0>(p), y = get<1>(p);
  return tuple<ll, ll>(x + y, x - y);
}

bool comp(const std::string &lh, const std::string &rh) {
  // 文字列lhの長さが文字列rhより短かったらtrueを返す
  if (lh.length() == rh.length())
    return lh > rh;
  else
    return lh.length() > rh.length();
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(20);

  string s;
  cin >> s;
  ll N;
  cin >> N;

  char now = 'z';
  REP(i, s.size()) { chmin(now, s[i]); }
  set<string> ans;
  while (true) {
    REP(i, s.size()) {
      if (s[i] == now) {
        ans.insert(s.substr(i, 1));
        int cnt = 0;
        int num = min((int)s.size(), i + 5);
        FOR(j, i + 1, num) {
          ans.insert(s.substr(i, 2 + cnt));
          cnt++;
        }
      }
    }

    now++;
    if (ans.size() >= N)
      break;
  }

  auto itr = ans.begin();
  int cnt = 1;
  while (cnt < N)
    itr++, cnt++;
  cout << *itr << endl;

  return 0;
}