Problem p03353 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

typedef long long ll;
typedef unsigned long long ull;
using namespace std;

#define FALSE printf("false\n");
#define TRUE printf("true\n");
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < ((int)(n)); ++i)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define test(a)                                                                \
  cout << "line:" << __LINE__ << "[" << (#a) << ": " << (a) << "]" << endl

const int INF = 1e9 + 7;
const ll INFL = 9 * 1e18;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

#define SUM(vec) accumulate(all(vec), 0LL) // 配列の中身を全部足す
template <typename T> void print(T &&x) { cout << x << endl; } // 改行付き出力
ll inline digit(ll num) {
  int tmp = 0;
  while (num) {
    tmp++;
    num /= 10;
  }
  return tmp;
} // 桁数
template <typename T> inline T digitSum(T num) {
  T sum = 0;
  while (num) {
    sum += num % 10;
    num /= 10;
  }
  return sum;
} // 各桁の和
template <typename T> inline T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
} // 最大公約数
template <typename T> inline T lcm(T a, T b) {
  T g = gcd(a, b);
  return a / g * b;
} // 最小公倍数
template <typename T> inline T power(T a, T b) {
  T tmp = 1;
  rep(i, b) { tmp *= a; }
  return tmp;
} // 累乗
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 ctoi(char c) { return c - '0'; }

// TODO: write
int main() {
  string s;
  cin >> s;
  int k;
  cin >> k;
  vector<string> str;
  rep(i, s.size() + 1) {
    for (int j = 1; j < s.size() + 1 - i; ++j) {
      str.pb(s.substr(i, j));
    }
  }
  sort(all(str));
  UNIQUE(str);
  print(str[k - 1]);
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

typedef long long ll;
typedef unsigned long long ull;
using namespace std;

#define FALSE printf("false\n");
#define TRUE printf("true\n");
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < ((int)(n)); ++i)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define test(a)                                                                \
  cout << "line:" << __LINE__ << "[" << (#a) << ": " << (a) << "]" << endl

const int INF = 1e9 + 7;
const ll INFL = 9 * 1e18;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

#define SUM(vec) accumulate(all(vec), 0LL) // 配列の中身を全部足す
template <typename T> void print(T &&x) { cout << x << endl; } // 改行付き出力
ll inline digit(ll num) {
  int tmp = 0;
  while (num) {
    tmp++;
    num /= 10;
  }
  return tmp;
} // 桁数
template <typename T> inline T digitSum(T num) {
  T sum = 0;
  while (num) {
    sum += num % 10;
    num /= 10;
  }
  return sum;
} // 各桁の和
template <typename T> inline T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
} // 最大公約数
template <typename T> inline T lcm(T a, T b) {
  T g = gcd(a, b);
  return a / g * b;
} // 最小公倍数
template <typename T> inline T power(T a, T b) {
  T tmp = 1;
  rep(i, b) { tmp *= a; }
  return tmp;
} // 累乗
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 ctoi(char c) { return c - '0'; }

// TODO: write
int main() {
  string s;
  cin >> s;
  int k;
  cin >> k;
  vector<string> str;
  rep(i, s.size() + 1) {
    for (int j = 1; j < s.size() + 1 - i && j <= k; ++j) {
      str.pb(s.substr(i, j));
    }
  }
  sort(all(str));
  UNIQUE(str);
  print(str[k - 1]);
  return 0;
}