Submission #1001145

---

Source Code Expand

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#endif

#include <bits/stdc++.h>

using namespace std;

mt19937 mrand(random_device{} ()); 

int rnd(int x) {
  return mrand() % x;
}

typedef long double ld;
typedef long long ll;

#ifdef DEBUG
#define eprintf(...) fprintf(stderr, __VA_ARGS__), fflush(stderr)
#else
#define eprintf(...) ;
#endif

#define pb push_back
#define mp make_pair
#define sz(x) ((int) (x).size())
#define TASK "text"

const int inf = (int) 1.01e9;
const ld eps = 1e-9;
const ld pi = acos((ld) -1.0);

const int mod = (int) 1e9 + 7;

void add(int &x, int y) {
  if ((x += y) >= mod) {
    x -= mod;
  }
}

int mult(int x, int y) {
  return (long long) x * y % mod;
}

int myPower(int x, int pw) {
  int res = 1;
  for (; pw; pw >>= 1) {
    if (pw & 1) {
      res = mult(res, x);
    }
    x = mult(x, x);
  }
  return res;
}

const int maxn = (int) 1e4 + 10;
int fact[maxn], ifact[maxn];

void precalc() {
  fact[0] = ifact[0] = 1;
  for (int i = 1; i < maxn; ++i) {
    fact[i] = mult(fact[i - 1], i);
    ifact[i] = mult(ifact[i - 1], myPower(i, mod - 2));
  }
}

int n, a, b, c;

int read() {
  if (scanf("%d%d%d%d", &n, &a, &b, &c) < 4) {
    return 0;
  }
  return 1;
}

void solve() {
  if (b & 1) {
    printf("0\n");
    return;
  }
  int b2 = b / 2;

  int res = 0;
  for (int x = 0; x <= a; ++x) {
    for (int y = 0;; ++y) {
      int left = c - (3 * y + a - x);
      if (left < 0) {
        break;
      }
      int cur = 1;
      cur = mult(cur, fact[a + y + b2]);
      cur = mult(cur, ifact[x]);
      cur = mult(cur, ifact[a - x]);
      cur = mult(cur, ifact[y]);
      cur = mult(cur, ifact[b2]);
      if (b2 == 0) {
        if (left != 0) {
          cur = 0;
        }
      } else {
        cur = mult(cur, fact[left + b2 - 1]);
        cur = mult(cur, ifact[b2 - 1]);
        cur = mult(cur, ifact[left]);
      }
      add(res, cur);
    }
  }
  printf("%d\n", res);
}

int main() {
  precalc();
#ifdef LOCAL
  freopen(TASK ".out", "w", stdout);
  assert(freopen(TASK ".in", "r", stdin));
#endif

  while (1) {
    if (!read()) {
      break;
    }
    solve();
#ifdef DEBUG
    eprintf("Time %.2f\n", (double) clock() / CLOCKS_PER_SEC);
#endif
  }
  return 0;
}

Submission Info

Judge Result