Problem p02744 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define rrng(a) a.rbegin(), a.rend()
#define uni(x) x.erase(unique(rng(x)), x.end())
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define dup(x, y) (((x) + (y)-1) / (y))
#define v(T) vector<T>
#define vv(T) v(v(T))
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pil = pair<int, ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
template <typename T> inline istream &operator>>(istream &i, v(T) & v) {
  rep(j, v.size()) i >> v[j];
  return i;
}
template <typename T> string join(const v(T) & v) {
  stringstream s;
  rep(i, v.size()) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T> inline ostream &operator<<(ostream &o, const v(T) & v) {
  if (v.size())
    o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream &operator>>(istream &i, pair<T1, T2> &v) {
  return i >> v.fi >> v.se;
}
template <typename T1, typename T2>
inline ostream &operator<<(ostream &o, const pair<T1, T2> &v) {
  return o << v.fi << "," << v.se;
}
const double eps = 1e-6;
const ll LINF = 100200300400500600ll;
const int INF = 1001001001;
//--------------------------------------------------------
#define fastio                                                                 \
  cin.tie(nullptr);                                                            \
  std::ios::sync_with_stdio(false);
#define myio                                                                   \
  ifstream iofi;                                                               \
  if (argc > 1) {                                                              \
    iofi.open(argv[1]);                                                        \
    std::cin.rdbuf(iofi.rdbuf());                                              \
  }
#define myloopset                                                              \
  int loop = 1;                                                                \
  if (argc > 2)                                                                \
    loop = stoi(argv[2]);                                                      \
  rep(i, loop)
//--------------------------------------------------------------
template <class T> T chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  } else
    return 0;
}
template <class T> T chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  } else
    return 0;
}
const int dx[4] = {0, 1, 0, -1};
const int dy[4] = {-1, 0, 1, 0};
const int mod = 1000000007;

void Main() {
  int n;
  cin >> n;
  vv(string) ans(10);
  ans[0].pb("a");
  rep(i, n) {
    for (auto s : ans[i]) {
      set<char> se;
      for (int sk = 0; sk < s.length(); sk++) {
        se.insert(s[sk]);
      }
      rep(j, se.size() + 1) {
        char c = 'a' + j;
        ans[i + 1].pb(s + (string){c});
      }
    }
  }
  sort(rng(ans[n - 1]));
  for (auto s : ans[n - 1]) {
    cout << s << endl;
  }
}
signed main(int argc, char **argv) {
  fastio;
  myio;
  myloopset Main();
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define rrng(a) a.rbegin(), a.rend()
#define uni(x) x.erase(unique(rng(x)), x.end())
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define dup(x, y) (((x) + (y)-1) / (y))
#define v(T) vector<T>
#define vv(T) v(v(T))
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pil = pair<int, ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
template <typename T> inline istream &operator>>(istream &i, v(T) & v) {
  rep(j, v.size()) i >> v[j];
  return i;
}
template <typename T> string join(const v(T) & v) {
  stringstream s;
  rep(i, v.size()) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T> inline ostream &operator<<(ostream &o, const v(T) & v) {
  if (v.size())
    o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream &operator>>(istream &i, pair<T1, T2> &v) {
  return i >> v.fi >> v.se;
}
template <typename T1, typename T2>
inline ostream &operator<<(ostream &o, const pair<T1, T2> &v) {
  return o << v.fi << "," << v.se;
}
const double eps = 1e-6;
const ll LINF = 100200300400500600ll;
const int INF = 1001001001;
//--------------------------------------------------------
#define fastio                                                                 \
  cin.tie(nullptr);                                                            \
  std::ios::sync_with_stdio(false);
#define myio                                                                   \
  ifstream iofi;                                                               \
  if (argc > 1) {                                                              \
    iofi.open(argv[1]);                                                        \
    std::cin.rdbuf(iofi.rdbuf());                                              \
  }
#define myloopset                                                              \
  int loop = 1;                                                                \
  if (argc > 2)                                                                \
    loop = stoi(argv[2]);                                                      \
  rep(i, loop)
//--------------------------------------------------------------
template <class T> T chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  } else
    return 0;
}
template <class T> T chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  } else
    return 0;
}
const int dx[4] = {0, 1, 0, -1};
const int dy[4] = {-1, 0, 1, 0};
const int mod = 1000000007;

void Main() {
  int n;
  cin >> n;
  vv(string) ans(11);
  ans[0].pb("a");
  rep(i, n) {
    for (auto s : ans[i]) {
      set<char> se;
      for (int sk = 0; sk < s.length(); sk++) {
        se.insert(s[sk]);
      }
      rep(j, se.size() + 1) {
        char c = 'a' + j;
        ans[i + 1].pb(s + (string){c});
      }
    }
  }
  sort(rng(ans[n - 1]));
  for (auto s : ans[n - 1]) {
    cout << s << endl;
  }
}
signed main(int argc, char **argv) {
  fastio;
  myio;
  myloopset Main();
}