Problem p03355 (C++)

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

template <class T1, class T2>
ostream &operator<<(ostream &out, const pair<T1, T2> p) {
  out << '(' << p.first << ',' << p.second << ')';
  return out;
}

template <class T1, class T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
  in >> p.first >> p.second;
  return in;
}

template <class T> istream &operator>>(istream &in, vector<T> &v) {
  for (T &x : v)
    in >> x;
  return in;
}

template <class T>
ostream &operator<<(ostream &out, const vector<vector<T>> &v) {
  for (const vector<T> &x : v)
    out << x << '\n';
  return out;
}

template <class T> ostream &operator<<(ostream &out, const vector<T> &v) {
  for (const T &x : v)
    out << x << ' ';
  return out;
}

#define ll long long
#define lf long double
#define pb push_back
#define mp make_pair
#define F first
#define S second
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;
typedef vector<bool> vb;
typedef vector<vector<ll>> vv;
typedef vector<vector<vector<ll>>> vvv;
typedef vector<string> vs;
typedef vector<lf> vf;
#define sz(x) (ll)(x).size()
#define tr1(x) cerr << #x << ": " << x << nl
#define tr2(x, y) cerr << #x << ": " << x << " | " << #y << ": " << y << nl
#define tr3(x, y, z)                                                           \
  cerr << #x << ": " << x << " | " << #y << ": " << y << " | " << #z << ": "   \
       << z << nl
#define tr4(x, y, z, w)                                                        \
  cerr << #x << ": " << x << " | " << #y << ": " << y << " | " << #z << ": "   \
       << z << " | " << #w << ": " << w << nl
#define trv(a)                                                                 \
  cerr << #a << ": ";                                                          \
  for (ll zz = 0; zz < sz(a); zz++)                                            \
    cerr << a[zz] << " ";                                                      \
  cerr << nl
#define trset(s)                                                               \
  cerr << #s << ": ";                                                          \
  for (auto &i : s)                                                            \
    cerr << i << " ";                                                          \
  cerr << nl
#define epsilon 0.000000001L
#define inf 9999999999999999ll // ~ 10^16
#define nl endl
#define all(x) x.begin(), x.end()
#define r_all(x) x.rbegin(), x.rend()
#define fast()                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL)
#define mod 1000000007ll
#define modulo 998244353ll
ll power(ll a, ll b) {
  return b ? power(a * a % mod, b / 2) * (b % 2 ? a : 1) % mod : 1;
}

int main() {
  fast();
  string s;
  ll k;
  cin >> s >> k;
  set<string> ans;
  for (ll i = 0; i < sz(s); i++) {
    string p;
    for (ll j = i; j < min(sz(s), j + k); j++) {
      p.pb(s[j]);
      ans.insert(p);
    }
  }
  auto it = ans.begin();
  for (ll i = 0; i < k - 1; i++)
    it++;
  cout << *it << nl;
}


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

template <class T1, class T2>
ostream &operator<<(ostream &out, const pair<T1, T2> p) {
  out << '(' << p.first << ',' << p.second << ')';
  return out;
}

template <class T1, class T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
  in >> p.first >> p.second;
  return in;
}

template <class T> istream &operator>>(istream &in, vector<T> &v) {
  for (T &x : v)
    in >> x;
  return in;
}

template <class T>
ostream &operator<<(ostream &out, const vector<vector<T>> &v) {
  for (const vector<T> &x : v)
    out << x << '\n';
  return out;
}

template <class T> ostream &operator<<(ostream &out, const vector<T> &v) {
  for (const T &x : v)
    out << x << ' ';
  return out;
}

#define ll long long
#define lf long double
#define pb push_back
#define mp make_pair
#define F first
#define S second
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<ii> vii;
typedef vector<bool> vb;
typedef vector<vector<ll>> vv;
typedef vector<vector<vector<ll>>> vvv;
typedef vector<string> vs;
typedef vector<lf> vf;
#define sz(x) (ll)(x).size()
#define tr1(x) cerr << #x << ": " << x << nl
#define tr2(x, y) cerr << #x << ": " << x << " | " << #y << ": " << y << nl
#define tr3(x, y, z)                                                           \
  cerr << #x << ": " << x << " | " << #y << ": " << y << " | " << #z << ": "   \
       << z << nl
#define tr4(x, y, z, w)                                                        \
  cerr << #x << ": " << x << " | " << #y << ": " << y << " | " << #z << ": "   \
       << z << " | " << #w << ": " << w << nl
#define trv(a)                                                                 \
  cerr << #a << ": ";                                                          \
  for (ll zz = 0; zz < sz(a); zz++)                                            \
    cerr << a[zz] << " ";                                                      \
  cerr << nl
#define trset(s)                                                               \
  cerr << #s << ": ";                                                          \
  for (auto &i : s)                                                            \
    cerr << i << " ";                                                          \
  cerr << nl
#define epsilon 0.000000001L
#define inf 9999999999999999ll // ~ 10^16
#define nl endl
#define all(x) x.begin(), x.end()
#define r_all(x) x.rbegin(), x.rend()
#define fast()                                                                 \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL)
#define mod 1000000007ll
#define modulo 998244353ll
ll power(ll a, ll b) {
  return b ? power(a * a % mod, b / 2) * (b % 2 ? a : 1) % mod : 1;
}

int main() {
  fast();
  string s;
  ll k;
  cin >> s >> k;
  set<string> ans;
  for (ll i = 0; i < sz(s); i++) {
    string p;
    for (ll j = i; j < min(sz(s), i + k); j++) {
      p.pb(s[j]);
      ans.insert(p);
    }
  }
  auto it = ans.begin();
  for (ll i = 0; i < k - 1; i++)
    it++;
  cout << *it << nl;
}