Problem p03353 (C++)

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

#define rep(i, n) for (int i = (int)(0); i < (int)(n); ++i)
#define irep(i, m, n) for (int i = (int)(m); i < (int)(n); ++i)
#define ireps(i, m, n) for (int i = (int)(m); i <= (int)(n); ++i)
#define FOR(e, c) for (auto &e : c)
#define SORT(v, n) sort(v, v + n);
#define vsort(v) sort(v.begin(), v.end());
#define rvisort(v) sort(v.begin(), v.end(), greater<int>());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cout(d) cout << d << endl;
#define coutd(d) cout << std::setprecision(10) << d << endl;
#define cinline(n) getline(cin, n);
#define replace_all(s, b, a) replace(s.begin(), s.end(), b, a);
// #define int long long
#define sz(x) int(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using ul = unsigned long;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

int GCD(int a, int b) { return b ? GCD(b, a % b) : a; }
int LCM(int a, int b) { return a * b / GCD(a, b); }

inline bool inside(int y, int x, int H, int W) {
  return (y >= 0 && x >= 0 && y < H && x < W);
}

const int INF = 1e9;
const int MOD = 1e9 + 7;
const ll LINF = 1e18;

signed main() {
  string s;
  cin >> s;
  int k;
  cin >> k;

  vector<string> a;
  rep(i, s.size()) {
    for (int j = i; j < s.size(); j++) {
      a.push_back(s.substr(i, (j - i + 1)));
    }
  }
  vsort(a);
  string pre = a[0];
  if (k == 1) {
    cout << pre << endl;
    return 0;
  }
  string ans;
  int cnt = 1;
  int i = 0;
  for (auto e : a) {
    i++;
    if (pre == a[i])
      continue;
    pre = a[i];
    cnt++;
    if (cnt == k) {
      ans = a[i];
      break;
    }
  }
  cout << ans << endl;
}


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

#define rep(i, n) for (int i = (int)(0); i < (int)(n); ++i)
#define irep(i, m, n) for (int i = (int)(m); i < (int)(n); ++i)
#define ireps(i, m, n) for (int i = (int)(m); i <= (int)(n); ++i)
#define FOR(e, c) for (auto &e : c)
#define SORT(v, n) sort(v, v + n);
#define vsort(v) sort(v.begin(), v.end());
#define rvisort(v) sort(v.begin(), v.end(), greater<int>());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cout(d) cout << d << endl;
#define coutd(d) cout << std::setprecision(10) << d << endl;
#define cinline(n) getline(cin, n);
#define replace_all(s, b, a) replace(s.begin(), s.end(), b, a);
// #define int long long
#define sz(x) int(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using ul = unsigned long;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

int GCD(int a, int b) { return b ? GCD(b, a % b) : a; }
int LCM(int a, int b) { return a * b / GCD(a, b); }

inline bool inside(int y, int x, int H, int W) {
  return (y >= 0 && x >= 0 && y < H && x < W);
}

const int INF = 1e9;
const int MOD = 1e9 + 7;
const ll LINF = 1e18;

signed main() {
  string s;
  cin >> s;
  int k;
  cin >> k;

  vector<string> a;
  rep(i, s.size()) {
    for (int j = i; j < min((int)s.size(), i + k); j++) {
      a.push_back(s.substr(i, (j - i + 1)));
    }
  }
  vsort(a);
  string pre = a[0];
  if (k == 1) {
    cout << pre << endl;
    return 0;
  }
  string ans;
  int cnt = 1;
  int i = 0;
  for (auto e : a) {
    i++;
    if (pre == a[i])
      continue;
    pre = a[i];
    cnt++;
    if (cnt == k) {
      ans = a[i];
      break;
    }
  }
  cout << ans << endl;
}