Problem p03355 (C++)

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

// conversion
//------------------------------------------
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

// math
//-------------------------------------------
template <class T> inline T sqr(T x) { return x * x; }

// typedef
//------------------------------------------
typedef long long LL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef vector<bool> VB;
typedef vector<VB> VVB;
typedef vector<double> VD;
typedef vector<VD> VVD;
typedef vector<string> VS;
typedef vector<VS> VVS;
typedef vector<char> VC;
typedef vector<VC> VVC;
typedef vector<PII> VPII;
typedef vector<PLL> VPLL;
typedef priority_queue<int> PQGI; // 大きい順
typedef priority_queue<int, VI, greater<int>> PQLI;
typedef priority_queue<PII> PQGP;
typedef priority_queue<PII, VPII, greater<int>> PQLP;

// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EB emplace_back
#define PB push_back
#define PF push_front
#define POB pop_back
#define POF pop_front
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define SORTR(c) sort((c).rbegin(), (c).rend())
#define LB lower_bound
#define UB upper_bound
#define NEXP next_permutation
#define FI first
#define SE second

// repetition
//------------------------------------------
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define FORR(i, a, b) for (int i = (b - 1); i >= a; i--)
#define REPR(i, n) FORR(i, 0, n)
#define BFOR(bit, a, b) for (int bit = (a); bit < (1 << (b)); ++bit)
#define BREP(bit, n) BFOR(bit, 0, n)

// constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int INF = INT_MAX / 2;
const LL LINF = LLONG_MAX / 3;
const int RINF = INT_MIN / 2;
const LL RLINF = LLONG_MIN / 3;
const LL MOD = 1e9 + 7;
const LL MODD = 998244353;
const int MAX = 510000;

inline bool Eq(double a, double b) { return fabs(b - a) < EPS; }

// other
//-------------------------------------------
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;
}

#define COUT(x) cout << (x) << endl
#define COUT2(x, y) cout << (x) << " " << (y) << endl
#define COUT3(x, y, z) cout << (x) << " " << (y) << " " << (z) << endl
#define PR(x) cout << (x)
#define ENDL cout << endl
#define SPACE cout << " "
#define BC(x) __builtin_popcountll(x)

void cYes() {
  COUT("Yes");
  exit(0);
}
void cNo() {
  COUT("No");
  exit(0);
}
void cYES() {
  COUT("YES");
  exit(0);
}
void cNO() {
  COUT("NO");
  exit(0);
}
void cyes() {
  COUT("yes");
  exit(0);
}
void cno() {
  COUT("no");
  exit(0);
}
void cMINU() {
  COUT(-1);
  exit(0);
}
void cZERO() {
  COUT(0);
  exit(0);
}

VI dx = {1, 0, -1, 0, 1, 1, -1, -1};
VI dy = {0, 1, 0, -1, 1, -1, 1, -1};
VC dc = {'R', 'D', 'L', 'U'};
VS ds = {"R", "D", "L", "U", "RD", "RU", "LD", "LU"};

LL Gcd(LL a, LL b) { return __gcd(a, b); }
LL Lcm(LL a, LL b) { return a / Gcd(a, b) * b; }

LL ModPow(LL A, LL N, LL M) {
  LL res = 1;
  while (N > 0) {
    if (N & 1)
      res = res * A % M;
    A = A * A % M;
    N >>= 1;
  }
  return res;
}

template <class T> void VCout(vector<T> &v) {
  int N = SZ(v);
  REP(i, N) {
    cout << v.at(i);
    if (i == N - 1)
      cout << endl;
    else
      cout << " ";
  }
  return;
}

int main() {

  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(12);

  string S;
  cin >> S;
  int N = SZ(S), K;
  cin >> K;
  set<string> Set;

  REP(i, N) {
    FOR(j, 1, N - i + 1) { Set.insert(S.substr(i, j)); }
  }
  auto it = Set.begin();
  advance(it, K - 1);
  COUT(*it);

  return 0;
}


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

// conversion
//------------------------------------------
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

// math
//-------------------------------------------
template <class T> inline T sqr(T x) { return x * x; }

// typedef
//------------------------------------------
typedef long long LL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef vector<bool> VB;
typedef vector<VB> VVB;
typedef vector<double> VD;
typedef vector<VD> VVD;
typedef vector<string> VS;
typedef vector<VS> VVS;
typedef vector<char> VC;
typedef vector<VC> VVC;
typedef vector<PII> VPII;
typedef vector<PLL> VPLL;
typedef priority_queue<int> PQGI; // 大きい順
typedef priority_queue<int, VI, greater<int>> PQLI;
typedef priority_queue<PII> PQGP;
typedef priority_queue<PII, VPII, greater<int>> PQLP;

// container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EB emplace_back
#define PB push_back
#define PF push_front
#define POB pop_back
#define POF pop_front
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c)                                                             \
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())
#define SORTR(c) sort((c).rbegin(), (c).rend())
#define LB lower_bound
#define UB upper_bound
#define NEXP next_permutation
#define FI first
#define SE second

// repetition
//------------------------------------------
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) FOR(i, 0, n)
#define FORR(i, a, b) for (int i = (b - 1); i >= a; i--)
#define REPR(i, n) FORR(i, 0, n)
#define BFOR(bit, a, b) for (int bit = (a); bit < (1 << (b)); ++bit)
#define BREP(bit, n) BFOR(bit, 0, n)

// constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int INF = INT_MAX / 2;
const LL LINF = LLONG_MAX / 3;
const int RINF = INT_MIN / 2;
const LL RLINF = LLONG_MIN / 3;
const LL MOD = 1e9 + 7;
const LL MODD = 998244353;
const int MAX = 510000;

inline bool Eq(double a, double b) { return fabs(b - a) < EPS; }

// other
//-------------------------------------------
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;
}

#define COUT(x) cout << (x) << endl
#define COUT2(x, y) cout << (x) << " " << (y) << endl
#define COUT3(x, y, z) cout << (x) << " " << (y) << " " << (z) << endl
#define PR(x) cout << (x)
#define ENDL cout << endl
#define SPACE cout << " "
#define BC(x) __builtin_popcountll(x)

void cYes() {
  COUT("Yes");
  exit(0);
}
void cNo() {
  COUT("No");
  exit(0);
}
void cYES() {
  COUT("YES");
  exit(0);
}
void cNO() {
  COUT("NO");
  exit(0);
}
void cyes() {
  COUT("yes");
  exit(0);
}
void cno() {
  COUT("no");
  exit(0);
}
void cMINU() {
  COUT(-1);
  exit(0);
}
void cZERO() {
  COUT(0);
  exit(0);
}

VI dx = {1, 0, -1, 0, 1, 1, -1, -1};
VI dy = {0, 1, 0, -1, 1, -1, 1, -1};
VC dc = {'R', 'D', 'L', 'U'};
VS ds = {"R", "D", "L", "U", "RD", "RU", "LD", "LU"};

LL Gcd(LL a, LL b) { return __gcd(a, b); }
LL Lcm(LL a, LL b) { return a / Gcd(a, b) * b; }

LL ModPow(LL A, LL N, LL M) {
  LL res = 1;
  while (N > 0) {
    if (N & 1)
      res = res * A % M;
    A = A * A % M;
    N >>= 1;
  }
  return res;
}

template <class T> void VCout(vector<T> &v) {
  int N = SZ(v);
  REP(i, N) {
    cout << v.at(i);
    if (i == N - 1)
      cout << endl;
    else
      cout << " ";
  }
  return;
}

int main() {

  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(12);

  string S;
  cin >> S;
  int N = SZ(S), K;
  cin >> K;
  set<string> Set;

  REP(i, N) {
    FOR(j, 1, min(K + 1, N - i + 1)) { Set.insert(S.substr(i, j)); }
  }
  auto it = Set.begin();
  advance(it, K - 1);
  COUT(*it);

  return 0;
}