Problem p03353 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <climits>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#define rep(i, m, n) for (int i = int(m); i < int(n); i++)
#define all(c) begin(c), end(c)

template <typename T1, typename T2> inline void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}

template <typename T1, typename T2> inline void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}

// 改造
typedef long long int ll;
using namespace std;
#define INF (1 << 30) - 1
#define INFl (ll)5e15
#define DEBUG 0 // デバッグする時1にしてね
#define dump(x) cerr << #x << " = " << (x) << endl
#define MOD 1000000007

// ここから編集する
class Solve {
public:
  string s;
  int K;

  void input() {
    cin >> s;
    cin >> K;
  }

  void solve() {
    input();

    int n = s.size();

    vector<vector<string>> vs(26);
    for (char c = 'a'; c <= 'z'; c++) {
      for (int i = 0; i < n; ++i) {
        if (s[i] == c) {
          for (int k = 1; i + k - 1 < n; ++k) {
            string tmp = s.substr(i, k);
            vs[c - 'a'].push_back(tmp);
          }
        }
      }
    }

    int rest = K;
    for (int i = 0; i < 26; ++i) {
      sort(all(vs[i]));
      vs[i].erase(unique(all(vs[i])), vs[i].end());
      if (vs[i].size() >= rest) {
        cout << vs[i][rest - 1] << endl;
        return;
      } else {
        rest -= vs[i].size();
      }
    }
  }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  Solve().solve();

  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <climits>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#define rep(i, m, n) for (int i = int(m); i < int(n); i++)
#define all(c) begin(c), end(c)

template <typename T1, typename T2> inline void chmin(T1 &a, T2 b) {
  if (a > b)
    a = b;
}

template <typename T1, typename T2> inline void chmax(T1 &a, T2 b) {
  if (a < b)
    a = b;
}

// 改造
typedef long long int ll;
using namespace std;
#define INF (1 << 30) - 1
#define INFl (ll)5e15
#define DEBUG 0 // デバッグする時1にしてね
#define dump(x) cerr << #x << " = " << (x) << endl
#define MOD 1000000007

// ここから編集する
class Solve {
public:
  string s;
  int K;

  void input() {
    cin >> s;
    cin >> K;
  }

  void solve() {
    input();

    int n = s.size();

    vector<vector<string>> vs(26);
    for (char c = 'a'; c <= 'z'; c++) {
      for (int i = 0; i < n; ++i) {
        if (s[i] == c) {
          for (int k = 1; i + k - 1 < n && k <= K; ++k) {
            string tmp = s.substr(i, k);
            vs[c - 'a'].push_back(tmp);
          }
        }
      }
    }

    int rest = K;
    for (int i = 0; i < 26; ++i) {
      sort(all(vs[i]));
      vs[i].erase(unique(all(vs[i])), vs[i].end());
      if (vs[i].size() >= rest) {
        cout << vs[i][rest - 1] << endl;
        return;
      } else {
        rest -= vs[i].size();
      }
    }
  }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  Solve().solve();

  return 0;
}