Problem p03353 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define BIT(n) (1LL << (n))
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPB(i, n) for (int i = 0; i < BIT(n); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); 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 FORA(i, n) for (auto &&i : n)
#define FORS(i, m, n) for (int i = m; i <= n; i++)
#define DEBUG(x) cerr << #x << ": " << x << "\n"
#define PRINTALL(V)                                                            \
  for (auto v : (V)) {                                                         \
    cout << v << " ";                                                          \
  }                                                                            \
  cout << "\n";
#define ALL(v) v.begin(), v.end()
#define fst first
#define snd second
#define mp make_pair
#define pb push_back
#define epb emplace_back
#define int long long
using namespace std;
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 (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> using vec = std::vector<T>;
template <class T> void print(const T &x) { cout << x << "\n"; }
const int MOD = 1000000007, INF = 1061109567;
const double EPS = 1e-10, PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
signed main() {
  cin.tie(0), ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  string S;
  int K;
  cin >> S >> K;
  int N = S.size();
  map<string, int> mp;
  REP(i, N) {
    FORS(j, i + 1, N) { mp[S.substr(i, j - i)]++; }
  }
  // DEBUG(mp.size());
  int cnt = 0;
  FORA(e, mp) {
    cnt++;
    if (cnt == K) {
      print(e.first);
      return 0;
    }
  }
}

Fixed submission:
#include <bits/stdc++.h>
#define BIT(n) (1LL << (n))
#define REP(i, n) for (int i = 0; i < n; i++)
#define REPB(i, n) for (int i = 0; i < BIT(n); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); 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 FORA(i, n) for (auto &&i : n)
#define FORS(i, m, n) for (int i = m; i <= n; i++)
#define DEBUG(x) cerr << #x << ": " << x << "\n"
#define PRINTALL(V)                                                            \
  for (auto v : (V)) {                                                         \
    cout << v << " ";                                                          \
  }                                                                            \
  cout << "\n";
#define ALL(v) v.begin(), v.end()
#define fst first
#define snd second
#define mp make_pair
#define pb push_back
#define epb emplace_back
#define int long long
using namespace std;
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 (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> using vec = std::vector<T>;
template <class T> void print(const T &x) { cout << x << "\n"; }
const int MOD = 1000000007, INF = 1061109567;
const double EPS = 1e-10, PI = acos(-1.0);
const int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
signed main() {
  cin.tie(0), ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
  string S;
  int K;
  cin >> S >> K;
  int N = S.size();
  map<string, int> mp;
  REP(i, N) {
    FORS(j, i + 1, i + 6) { mp[S.substr(i, j - i)]++; }
  }
  // DEBUG(mp.size());
  int cnt = 0;
  FORA(e, mp) {
    cnt++;
    if (cnt == K) {
      print(e.first);
      return 0;
    }
  }
}