Problem p02738 (C++)

Buggy submission (status: Runtime Error):
/*{{{*/
#include <bits/stdc++.h>
#define SZ(X) ((int)(X).size())
#define ALL(X) (X).begin(), (X).end()
#define REP(I, N) for (int I = 0; I < (N); ++I)
#define REPP(I, A, B) for (int I = (A); I < (B); ++I)
#define FOR(I, A, B) for (int I = (A); I <= (B); ++I)
#define FORS(I, S) for (int I = 0; S[I]; ++I)
#define RS(X) scanf("%s", (X))
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
#define CASET                                                                  \
  int ___T;                                                                    \
  scanf("%d", &___T);                                                          \
  for (int cs = 1; cs <= ___T; cs++)
#define MP make_pair
#define PB emplace_back
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define LEN(X) strlen(X)
#define F first
#define S second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int, int> PII;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<PII> VPII;
typedef pair<LL, LL> PLL;
typedef vector<PLL> VPLL;
template <class T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(int64_t &x) { scanf("%lld", &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }
void R() {}
template <class T, class... U> void R(T &head, U &...tail) {
  _R(head);
  R(tail...);
}
template <class T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
void _W(const int64_t &x) { printf("%lld", x); }
void _W(const double &x) { printf("%.16f", x); }
void _W(const char &x) { putchar(x); }
void _W(const char *x) { printf("%s", x); }
template <class T, class U> void _W(const pair<T, U> &x) {
  _W(x.F);
  putchar(' ');
  _W(x.S);
}
template <class T> void _W(const vector<T> &x) {
  for (auto i = x.begin(); i != x.end(); _W(*i++))
    if (i != x.cbegin())
      putchar(' ');
}
void W() {}
template <class T, class... U> void W(const T &head, const U &...tail) {
  _W(head);
  putchar(sizeof...(tail) ? ' ' : '\n');
  W(tail...);
}
#ifdef HOME
#define DEBUG(...)                                                             \
  {                                                                            \
    printf("# ");                                                              \
    printf(__VA_ARGS__);                                                       \
    puts("");                                                                  \
  }
#else
#define DEBUG(...)
#endif
int MOD = 1e9 + 7;
void ADD(LL &x, LL v) {
  x = (x + v) % MOD;
  if (x < 0)
    x += MOD;
}
/*}}}*/
const int SIZE = 6001;
int N;
LL dp[SIZE][2001][2];
LL fac[SIZE], inv[SIZE];
LL mypow(LL x, LL y) {
  LL res = 1;
  while (y) {
    if (y & 1)
      res = res * x % MOD;
    y >>= 1;
    x = x * x % MOD;
  }
  return res;
}
LL C(int x, int y) {
  if (y < 0 || y > x)
    return 0;
  return fac[x] * inv[y] % MOD * inv[x - y] % MOD;
}
void build() {
  assert(MOD >= SIZE);
  fac[0] = 1;
  REPP(i, 1, SIZE) fac[i] = fac[i - 1] * i % MOD;
  inv[SIZE - 1] = mypow(fac[SIZE - 1], MOD - 2);
  for (int i = SIZE - 2; i >= 0; i--)
    inv[i] = inv[i + 1] * (i + 1) % MOD;
}
int get(int x, int y) {
  if (y == 1)
    return x;
  return (x + 2) / 3;
}
LL calc(int n, int x, int y) {
  if (n > 3 * N)
    return 0;
  if (n == 3 * N)
    return 1;
  if (dp[n][x][y] != -1)
    return dp[n][x][y];
  LL &ret = dp[n][x][y];
  ret = 0;
  int use3 = (n - (y + 1) * x) / 3;
  if (use3 + 1 + get(x, y) <= N)
    ADD(ret, calc(n + 3, x, y) * (3 * N - n - 1LL) % MOD * (3 * N - n - 2LL));
  if (y == 1 || x == 0) {
    if (use3 + get(x + 1, 1) <= N)
      ADD(ret, calc(n + 2, x + 1, 1) * (3 * N - n - 1LL));
  } else {
    if (use3 + 1 + get(x - 1, 0) <= N)
      ADD(ret, calc(n + 2, x - 1, 0) * (3 * N - n - 1LL));
  }
  if (y == 0 || x == 0) {
    if (use3 + get(x + 1, 0) <= N)
      ADD(ret, calc(n + 1, x + 1, 0));
  } else {
    if (use3 + 1 + get(x - 1, 1) <= N)
      ADD(ret, calc(n + 1, x - 1, 1));
  }
  return ret;
}
int main() {
  build();
  MS1(dp);
  R(N, MOD);
  W(calc(0, 0, 0));
  return 0;
}


Fixed submission:
/*{{{*/
#include <bits/stdc++.h>
#define SZ(X) ((int)(X).size())
#define ALL(X) (X).begin(), (X).end()
#define REP(I, N) for (int I = 0; I < (N); ++I)
#define REPP(I, A, B) for (int I = (A); I < (B); ++I)
#define FOR(I, A, B) for (int I = (A); I <= (B); ++I)
#define FORS(I, S) for (int I = 0; S[I]; ++I)
#define RS(X) scanf("%s", (X))
#define SORT_UNIQUE(c)                                                         \
  (sort(c.begin(), c.end()),                                                   \
   c.resize(distance(c.begin(), unique(c.begin(), c.end()))))
#define GET_POS(c, x) (lower_bound(c.begin(), c.end(), x) - c.begin())
#define CASET                                                                  \
  int ___T;                                                                    \
  scanf("%d", &___T);                                                          \
  for (int cs = 1; cs <= ___T; cs++)
#define MP make_pair
#define PB emplace_back
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define LEN(X) strlen(X)
#define F first
#define S second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int, int> PII;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<PII> VPII;
typedef pair<LL, LL> PLL;
typedef vector<PLL> VPLL;
template <class T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(int64_t &x) { scanf("%lld", &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }
void R() {}
template <class T, class... U> void R(T &head, U &...tail) {
  _R(head);
  R(tail...);
}
template <class T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
void _W(const int64_t &x) { printf("%lld", x); }
void _W(const double &x) { printf("%.16f", x); }
void _W(const char &x) { putchar(x); }
void _W(const char *x) { printf("%s", x); }
template <class T, class U> void _W(const pair<T, U> &x) {
  _W(x.F);
  putchar(' ');
  _W(x.S);
}
template <class T> void _W(const vector<T> &x) {
  for (auto i = x.begin(); i != x.end(); _W(*i++))
    if (i != x.cbegin())
      putchar(' ');
}
void W() {}
template <class T, class... U> void W(const T &head, const U &...tail) {
  _W(head);
  putchar(sizeof...(tail) ? ' ' : '\n');
  W(tail...);
}
#ifdef HOME
#define DEBUG(...)                                                             \
  {                                                                            \
    printf("# ");                                                              \
    printf(__VA_ARGS__);                                                       \
    puts("");                                                                  \
  }
#else
#define DEBUG(...)
#endif
int MOD = 1e9 + 7;
void ADD(LL &x, LL v) {
  x = (x + v) % MOD;
  if (x < 0)
    x += MOD;
}
/*}}}*/
const int SIZE = 6001;
int N;
LL dp[SIZE][SIZE][2];
LL fac[SIZE], inv[SIZE];
LL mypow(LL x, LL y) {
  LL res = 1;
  while (y) {
    if (y & 1)
      res = res * x % MOD;
    y >>= 1;
    x = x * x % MOD;
  }
  return res;
}
LL C(int x, int y) {
  if (y < 0 || y > x)
    return 0;
  return fac[x] * inv[y] % MOD * inv[x - y] % MOD;
}
void build() {
  assert(MOD >= SIZE);
  fac[0] = 1;
  REPP(i, 1, SIZE) fac[i] = fac[i - 1] * i % MOD;
  inv[SIZE - 1] = mypow(fac[SIZE - 1], MOD - 2);
  for (int i = SIZE - 2; i >= 0; i--)
    inv[i] = inv[i + 1] * (i + 1) % MOD;
}
int get(int x, int y) {
  if (y == 1)
    return x;
  return (x + 2) / 3;
}
LL calc(int n, int x, int y) {
  if (n > 3 * N)
    return 0;
  if (n == 3 * N)
    return 1;
  if (dp[n][x][y] != -1)
    return dp[n][x][y];
  LL &ret = dp[n][x][y];
  ret = 0;
  int use3 = (n - (y + 1) * x) / 3;
  if (use3 + 1 + get(x, y) <= N)
    ADD(ret, calc(n + 3, x, y) * (3 * N - n - 1LL) % MOD * (3 * N - n - 2LL));
  if (y == 1 || x == 0) {
    if (use3 + get(x + 1, 1) <= N)
      ADD(ret, calc(n + 2, x + 1, 1) * (3 * N - n - 1LL));
  } else {
    if (use3 + 1 + get(x - 1, 0) <= N)
      ADD(ret, calc(n + 2, x - 1, 0) * (3 * N - n - 1LL));
  }
  if (y == 0 || x == 0) {
    if (use3 + get(x + 1, 0) <= N)
      ADD(ret, calc(n + 1, x + 1, 0));
  } else {
    if (use3 + 1 + get(x - 1, 1) <= N)
      ADD(ret, calc(n + 1, x - 1, 1));
  }
  return ret;
}
int main() {
  build();
  MS1(dp);
  R(N, MOD);
  W(calc(0, 0, 0));
  return 0;
}