Problem p03353 (C++)

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

struct Fast {
  Fast() {
    std::cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;

/* cpp template {{{ */

/* short */
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define Fi first
#define Se second
#define ALL(v) begin(v), end(v)
#define RALL(v) (v).rbegin(), (v).rend()
#define X real()
#define Y imag()

/* REPmacro */
#define REPS(i, a, n) for (ll i = (a); i < (ll)(n); ++i)
#define rep(i, n) REPS(i, 0, n)
#define REP1(i, n) for (ll i = 1; i <= (ll)(n); ++i)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (ll i = (a); i >= (ll)(n); --i)
#define DEP(i, n) DEPS(i, n, 0)
#define EACH(i, n) for (auto &&i : n)

/* debug */
#define debug(x)                                                               \
  cerr << x << " "                                                             \
       << "(L:" << __LINE__ << ")" << '\n';

/* alias */
using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using D = double;
using P = complex<D>;
using vs = vector<string>;
template <typename T> using PQ = priority_queue<T>;
template <typename T> using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const int INF = 1001001001;
const ll LINF = 1001001001001001001ll;
const int MOD = 1e9 + 7;
const D EPS = 1e-9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1},
          dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

/* func */
inline bool inside(int y, int x, int H, int W) {
  return y >= 0 && x >= 0 && y < H && x < W;
}
inline int in() {
  int x;
  cin >> x;
  return x;
}
inline ll IN() {
  ll x;
  cin >> x;
  return x;
}
inline vs split(const string &t, char c) {
  vs v;
  stringstream s(t);
  string b;
  while (getline(s, b, c))
    v.eb(b);
  return v;
}
template <typename T> inline bool chmin(T &a, const T &b) {
  if (a > b)
    a = b;
  return a > b;
}
template <typename T> inline bool chmax(T &a, const T &b) {
  if (a < b)
    a = b;
  return a < b;
}
template <typename T, typename S> inline void print(const pair<T, S> &p) {
  cout << p.first << " " << p.second << endl;
}
template <typename T> inline void print(const T &x) { cout << x << '\n'; }
template <typename T, typename S>
inline void print(const vector<pair<T, S>> &v) {
  for (auto &&p : v)
    print(p);
}
template <typename T> inline void print(const vector<T> &v, string s = " ") {
  rep(i, v.size()) cout << v[i] << (i != (ll)v.size() - 1 ? s : "\n");
}

signed main() {
  string s;
  cin >> s;
  int k = in();
  set<string> ss;
  rep(i, s.size()) {
    REPS(j, i, s.size()) { ss.insert(s.substr(i, j - i + 1)); }
  }
  auto itr = ss.begin();
  rep(i, k - 1)++ itr;
  print(*itr);

  return 0;
}

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

struct Fast {
  Fast() {
    std::cin.tie(0);
    ios::sync_with_stdio(false);
  }
} fast;

/* cpp template {{{ */

/* short */
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define Fi first
#define Se second
#define ALL(v) begin(v), end(v)
#define RALL(v) (v).rbegin(), (v).rend()
#define X real()
#define Y imag()

/* REPmacro */
#define REPS(i, a, n) for (ll i = (a); i < (ll)(n); ++i)
#define rep(i, n) REPS(i, 0, n)
#define REP1(i, n) for (ll i = 1; i <= (ll)(n); ++i)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (ll i = (a); i >= (ll)(n); --i)
#define DEP(i, n) DEPS(i, n, 0)
#define EACH(i, n) for (auto &&i : n)

/* debug */
#define debug(x)                                                               \
  cerr << x << " "                                                             \
       << "(L:" << __LINE__ << ")" << '\n';

/* alias */
using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using D = double;
using P = complex<D>;
using vs = vector<string>;
template <typename T> using PQ = priority_queue<T>;
template <typename T> using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const int INF = 1001001001;
const ll LINF = 1001001001001001001ll;
const int MOD = 1e9 + 7;
const D EPS = 1e-9;
const int dx[] = {0, 1, 0, -1, 1, -1, 1, -1},
          dy[] = {1, 0, -1, 0, 1, -1, -1, 1};

/* func */
inline bool inside(int y, int x, int H, int W) {
  return y >= 0 && x >= 0 && y < H && x < W;
}
inline int in() {
  int x;
  cin >> x;
  return x;
}
inline ll IN() {
  ll x;
  cin >> x;
  return x;
}
inline vs split(const string &t, char c) {
  vs v;
  stringstream s(t);
  string b;
  while (getline(s, b, c))
    v.eb(b);
  return v;
}
template <typename T> inline bool chmin(T &a, const T &b) {
  if (a > b)
    a = b;
  return a > b;
}
template <typename T> inline bool chmax(T &a, const T &b) {
  if (a < b)
    a = b;
  return a < b;
}
template <typename T, typename S> inline void print(const pair<T, S> &p) {
  cout << p.first << " " << p.second << endl;
}
template <typename T> inline void print(const T &x) { cout << x << '\n'; }
template <typename T, typename S>
inline void print(const vector<pair<T, S>> &v) {
  for (auto &&p : v)
    print(p);
}
template <typename T> inline void print(const vector<T> &v, string s = " ") {
  rep(i, v.size()) cout << v[i] << (i != (ll)v.size() - 1 ? s : "\n");
}

signed main() {
  string s;
  cin >> s;
  int k = in();
  set<string> ss;
  rep(i, s.size()) {
    REPS(j, 1, 7) { ss.emplace(s.substr(i, j)); }
  }
  auto itr = ss.begin();
  rep(i, k - 1)++ itr;
  print(*itr);

  return 0;
}