Problem p03421 (C++)

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

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for (ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for (ll i = (a); i > (b); --i)
#define REP(i, n) for (ll i = 0; i < (n); ++i)
#define REPR(i, n) for (ll i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
#define VECCIN(x)                                                              \
  for (auto &youso_ : (x))                                                     \
  cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()

template <typename T = long long> inline T IN() {
  T x;
  cin >> x;
  return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&...tail) {
  cin >> head;
  CIN(move(tail)...);
}
#define CCIN(...)                                                              \
  char __VA_ARGS__;                                                            \
  CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
  double __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
  ll __VA_ARGS__;                                                              \
  CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
  string __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
  {                                                                            \
    FOREACH(x, v) { cout << x << " "; }                                        \
    cout << "\n";                                                              \
  }
template <typename T = string> inline void eputs(T s) {
  cout << s << "\n";
  exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
// const double PI = atan(1.0) * 4.0;
const ll dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

signed main() {
  LCIN(N, A, B);
  if (A + B > N + 1 || A * B < N)
    eputs(-1);
  VVL ans(A);
  ll now = 1;
  REP(i, B) { ans[0].push_back(now++); }
  REP(i, A - 1) { ans[i + 1].push_back(now++); }
  ll idx = 1;
  while (now <= N) {
    ans[idx].emplace_back(now);
    if (ans[idx].size() == B)
      idx++;
  }
  REP(i, A) REPR(j, ans[i].size() - 1) { cout << ans[i][j] << " "; }
  cout << "\n";
}


Fixed submission:
#include <bits/stdc++.h>

using namespace std;

#define int long long
// #define double long double
#define FOR(i, a, b) for (ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for (ll i = (a); i > (b); --i)
#define REP(i, n) for (ll i = 0; i < (n); ++i)
#define REPR(i, n) for (ll i = n; i >= 0; i--)
#define FOREACH(x, a) for (auto &(x) : (a))
#define VECCIN(x)                                                              \
  for (auto &youso_ : (x))                                                     \
  cin >> youso_
#define bitcnt(x) __builtin_popcount(x)
#define lbit(x) __builtin_ffsll(x)
#define rbit(x) __builtin_clzll(x)
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
#define rAll(a) (a).rbegin(), (a).rend()

template <typename T = long long> inline T IN() {
  T x;
  cin >> x;
  return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&...tail) {
  cin >> head;
  CIN(move(tail)...);
}
#define CCIN(...)                                                              \
  char __VA_ARGS__;                                                            \
  CIN(__VA_ARGS__)
#define DCIN(...)                                                              \
  double __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
  ll __VA_ARGS__;                                                              \
  CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
  string __VA_ARGS__;                                                          \
  CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
  {                                                                            \
    FOREACH(x, v) { cout << x << " "; }                                        \
    cout << "\n";                                                              \
  }
template <typename T = string> inline void eputs(T s) {
  cout << s << "\n";
  exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}

template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<ll, ll> PL;
typedef vector<PL> VPL;
typedef vector<bool> VB;
typedef vector<double> VD;
typedef vector<string> VS;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18;
// const double PI = atan(1.0) * 4.0;
const ll dx[] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[] = {0, 1, 1, 1, 0, -1, -1, -1};
#define PI 3.141592653589793238

signed main() {
  LCIN(N, A, B);
  if (A + B > N + 1 || A * B < N)
    eputs(-1);
  VVL ans(A);
  ll now = 1;
  REP(i, B) { ans[0].push_back(now++); }
  REP(i, A - 1) { ans[i + 1].push_back(now++); }
  ll idx = 1;
  while (now <= N) {
    ans[idx].emplace_back(now++);
    if (ans[idx].size() == B)
      idx++;
  }
  REP(i, A) REPR(j, ans[i].size() - 1) { cout << ans[i][j] << " "; }
  cout << "\n";
}