Problem p03353 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#include <type_traits>
using namespace std;

using ll = int64_t;
#define int ll

#define FOR(i, a, b) for (int i = int(a); i < int(b); i++)
#define REP(i, b) FOR(i, 0, b)
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define ALL(x) x.begin(), x.end()

using pi = pair<int, int>;
using vi = vector<int>;
using ld = long double;

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  REP(i, (int)v.size()) {
    if (i)
      os << ",";
    os << v[i];
  }
  os << "}";
  return os;
}

ll read() {
  ll i;
  scanf("%" SCNd64, &i);
  return i;
}

void printSpace() { printf(" "); }

void printEoln() { printf("\n"); }

void print(ll x, int suc = 1) {
  printf("%" PRId64, x);
  if (suc == 1)
    printEoln();
  if (suc == 2)
    printSpace();
}

string readString() {
  static char buf[3341000];
  scanf("%s", buf);
  return string(buf);
}

char *readCharArray() {
  static char buf[3341000];
  static int bufUsed = 0;
  char *ret = buf + bufUsed;
  scanf("%s", ret);
  bufUsed += strlen(ret) + 1;
  return ret;
}

template <class T, class U> void chmax(T &a, U b) {
  if (a < b)
    a = b;
}

template <class T, class U> void chmin(T &a, U b) {
  if (b < a)
    a = b;
}

template <class T> T Sq(const T &t) { return t * t; }

signed main() {
  string s;
  int k;
  cin >> s >> k;
  set<string> ss;
  for (int i = 0; i < s.length(); i++) {
    for (int j = 1; j <= 5; j++) {
      if (i + j >= s.length())
        continue;
      string str = s.substr(i, j);
      if (ss.size() <= k || *ss.rbegin() > str)
        ss.insert(str);
      while (ss.size() > k)
        ss.erase(--ss.end());
    }
  }
  set<string>::iterator it = ss.begin();
  advance(it, k - 1);
  cout << *it << endl;
}


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

using ll = int64_t;
#define int ll

#define FOR(i, a, b) for (int i = int(a); i < int(b); i++)
#define REP(i, b) FOR(i, 0, b)
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define ALL(x) x.begin(), x.end()

using pi = pair<int, int>;
using vi = vector<int>;
using ld = long double;

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  REP(i, (int)v.size()) {
    if (i)
      os << ",";
    os << v[i];
  }
  os << "}";
  return os;
}

ll read() {
  ll i;
  scanf("%" SCNd64, &i);
  return i;
}

void printSpace() { printf(" "); }

void printEoln() { printf("\n"); }

void print(ll x, int suc = 1) {
  printf("%" PRId64, x);
  if (suc == 1)
    printEoln();
  if (suc == 2)
    printSpace();
}

string readString() {
  static char buf[3341000];
  scanf("%s", buf);
  return string(buf);
}

char *readCharArray() {
  static char buf[3341000];
  static int bufUsed = 0;
  char *ret = buf + bufUsed;
  scanf("%s", ret);
  bufUsed += strlen(ret) + 1;
  return ret;
}

template <class T, class U> void chmax(T &a, U b) {
  if (a < b)
    a = b;
}

template <class T, class U> void chmin(T &a, U b) {
  if (b < a)
    a = b;
}

template <class T> T Sq(const T &t) { return t * t; }

signed main() {
  string s;
  int k;
  cin >> s >> k;
  set<string> ss;
  for (int i = 0; i < s.length(); i++) {
    for (int j = 1; j <= 5; j++) {
      if (i + j > s.length())
        break;
      ss.insert(s.substr(i, j));
    }
  }
  set<string>::iterator it = ss.begin();
  advance(it, k - 1);
  cout << *it << endl;
}