Problem p03355 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <ciso646>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define REP(i, n) for (int i = 0; i < (n); i++)
#define RREP(i, n) for (int i = (n)-1; i >= 0; i--)

#define inf 0x3f3f3f3f3f3f3f3f
#define PB push_back
#define MP make_pair
#define ALL(a) (a).begin(), (a).end()
#define SET(a, c) memset(a, c, sizeof a)
#define CLR(a) memset(a, 0, sizeof a)
#define VS vector<string>
#define VI vector<ll>
#define DEBUG(x) cout << #x << ": " << x << endl
#define MIN(a, b) (a > b ? b : a)
#define MAX(a, b) (a > b ? a : b)
#define pi 2 * acos(0.0)
#define INFILE() freopen("in0.txt", "r", stdin)
#define OUTFILE() freopen("out0.txt", "w", stdout)
#define ll long long
#define ull unsigned long long
#define pii pair<ll, ll>
#define pcc pair<char, char>
#define pic pair<ll, char>
#define pci pair<char, ll>
#define eps 1e-14
#define FST first
#define SEC second
#define SETUP                                                                  \
  cin.tie(0), ios::sync_with_stdio(false),                                     \
      cout << setprecision(15) << std::fixed;

namespace {
struct input_returnner {
  ll N;
  input_returnner(ll N_ = 0) : N(N_) {}
  template <typename T> operator vector<T>() const {
    vector<T> res(N);
    for (auto &a : res)
      cin >> a;
    return std::move(res);
  }
  template <typename T> operator T() const {
    T res;
    cin >> res;
    return res;
  }
  template <typename T> T operator-(T right) {
    return T(input_returnner()) - right;
  }
  template <typename T> T operator+(T right) {
    return T(input_returnner()) + right;
  }
  template <typename T> T operator*(T right) {
    return T(input_returnner()) * right;
  }
  template <typename T> T operator/(T right) {
    return T(input_returnner()) / right;
  }
  template <typename T> T operator<<(T right) {
    return T(input_returnner()) << right;
  }
  template <typename T> T operator>>(T right) {
    return T(input_returnner()) >> right;
  }
};
template <typename T> input_returnner in() { return in<T>(); }
input_returnner in() { return input_returnner(); }
input_returnner in(ll N) { return std::move(input_returnner(N)); }
} // namespace

template <typename T> struct is_vector : std::false_type {};

template <typename T> struct is_vector<std::vector<T>> : std::true_type {};

template <typename T> constexpr bool is_vector_v = is_vector<T>::value;

template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &v) {
  if (!v.empty()) {
    for (int i = 0; i < v.size(); ++i) {
      out << v[i] << (i == v.size() - 1 ? "\n" : (is_vector_v<T> ? "" : ", "));
    }
  }
  return out;
}

ll MOD = 1e9 + 7;

class prime;

void solve();

signed main() {
  SETUP;
  solve();
#ifdef _DEBUG
  system("pause");
#endif
  return 0;
}

#define int ll

void solve() {
  set<string> ss;
  string s;
  cin >> s;
  int K;
  cin >> K;
  for (int i = 0; i < s.size(); ++i) {
    for (int j = 0; j < s.size() - j; ++j) {
      ss.insert(s.substr(j, i + 1));
    }
  }
  auto it = ss.begin();
  for (int i = 0; i < K - 1; ++i)
    ++it;
  cout << *it << endl;
}


Fixed submission:
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <ciso646>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define REP(i, n) for (int i = 0; i < (n); i++)
#define RREP(i, n) for (int i = (n)-1; i >= 0; i--)

#define inf 0x3f3f3f3f3f3f3f3f
#define PB push_back
#define MP make_pair
#define ALL(a) (a).begin(), (a).end()
#define SET(a, c) memset(a, c, sizeof a)
#define CLR(a) memset(a, 0, sizeof a)
#define VS vector<string>
#define VI vector<ll>
#define DEBUG(x) cout << #x << ": " << x << endl
#define MIN(a, b) (a > b ? b : a)
#define MAX(a, b) (a > b ? a : b)
#define pi 2 * acos(0.0)
#define INFILE() freopen("in0.txt", "r", stdin)
#define OUTFILE() freopen("out0.txt", "w", stdout)
#define ll long long
#define ull unsigned long long
#define pii pair<ll, ll>
#define pcc pair<char, char>
#define pic pair<ll, char>
#define pci pair<char, ll>
#define eps 1e-14
#define FST first
#define SEC second
#define SETUP                                                                  \
  cin.tie(0), ios::sync_with_stdio(false),                                     \
      cout << setprecision(15) << std::fixed;

namespace {
struct input_returnner {
  ll N;
  input_returnner(ll N_ = 0) : N(N_) {}
  template <typename T> operator vector<T>() const {
    vector<T> res(N);
    for (auto &a : res)
      cin >> a;
    return std::move(res);
  }
  template <typename T> operator T() const {
    T res;
    cin >> res;
    return res;
  }
  template <typename T> T operator-(T right) {
    return T(input_returnner()) - right;
  }
  template <typename T> T operator+(T right) {
    return T(input_returnner()) + right;
  }
  template <typename T> T operator*(T right) {
    return T(input_returnner()) * right;
  }
  template <typename T> T operator/(T right) {
    return T(input_returnner()) / right;
  }
  template <typename T> T operator<<(T right) {
    return T(input_returnner()) << right;
  }
  template <typename T> T operator>>(T right) {
    return T(input_returnner()) >> right;
  }
};
template <typename T> input_returnner in() { return in<T>(); }
input_returnner in() { return input_returnner(); }
input_returnner in(ll N) { return std::move(input_returnner(N)); }
} // namespace

template <typename T> struct is_vector : std::false_type {};

template <typename T> struct is_vector<std::vector<T>> : std::true_type {};

template <typename T> constexpr bool is_vector_v = is_vector<T>::value;

template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &v) {
  if (!v.empty()) {
    for (int i = 0; i < v.size(); ++i) {
      out << v[i] << (i == v.size() - 1 ? "\n" : (is_vector_v<T> ? "" : ", "));
    }
  }
  return out;
}

ll MOD = 1e9 + 7;

class prime;

void solve();

signed main() {
  SETUP;
  solve();
#ifdef _DEBUG
  system("pause");
#endif
  return 0;
}

#define int ll

void solve() {
  set<string> ss;
  string s;
  cin >> s;
  int K;
  cin >> K;
  for (int i = 0; i < K; ++i) {
    for (int j = 0; j < s.size() - i; ++j) {
      ss.insert(s.substr(j, i + 1));
    }
  }
  auto it = ss.begin();
  for (int i = 0; i < K - 1; ++i)
    ++it;
  cout << *it << endl;
}