Submission #3519088

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Source Code Expand

#include <bits/stdc++.h>
#define rep(i, n) for (rint i = 1; i <= (n); i ++)
#define re0(i, n) for (rint i = 0; i < (int) n; i ++)
#define travel(i, u) for (rint i = head[u]; i; i = e[i].nxt)
#define rint register int
using namespace std;
 
template<typename tp> inline void read(tp &x) {
  x = 0; char c = getchar(); int f = 0;
  for (; c < '0' || c > '9'; f |= c == '-', c = getchar());
  for (; c >= '0' && c <= '9'; x = (x << 3) + (x << 1) + c - '0', c = getchar());
  if (f) x = -x;
}
namespace {
  const int mo = 1e9 + 7;
  inline int add(int x, int y) {
    if((x += y) >= mo) x -= mo;
    return x;
  }
  inline int sub(int x, int y) {
    if((x -= y) < 0) x += mo;
    return x;
  }
  inline int mul(int x, int y) {
    return 1LL * x * y % mo;
  }
  inline int pw(int a, int k) {
    int ans = 1;
    for (; k; k >>= 1, a = mul(a, a))
      if (k & 1) ans = mul(ans, a);
    return ans;
  }
  inline int inv(int n) {
    return pw(n, mo - 2);
  }
}

namespace binom {
  const int NN = 3e5 + 233;
  int fac[NN], ifac[NN];
  inline void init(void) {
    int m = 3e5;
    fac[0] = 1;
    for (int i = 1; i <= m; i ++)
      fac[i] = mul(fac[i - 1], i);
    ifac[m] = inv(fac[m]);
    for (int i = m - 1; i >= 0; i --)
      ifac[i] = mul(ifac[i + 1], i + 1);
  }

  inline int Bi(int n, int m) {
    if (n < 0) return 0;
    else if (m == n || m == 0) return 1;
    else return mul(fac[n], mul(ifac[m], ifac[n - m]));
  }
}
using namespace binom;
int A, B, C, N, ans;

/*
  1.1
  2.31
  3.333
  4.22
  4.2332
  4.23332
*/
inline int cao(int a, int b, int c, int d) {
  return mul(mul(Bi(a + b + c + d, a), Bi(b + c + d, b)),
             mul(Bi(c + d, c), Bi(d, d)));
}

int main(void) {
  init();
  read(N); read(A); read(B); read(C);
  for (int two = 0; two <= A; two ++)
    for (int three = 0; three <= C / 3; three ++) {
      int one = A - two;
      if (B % 2 == 1) continue;
      int four = B / 2;
      int resC = C - three * 3 - two;
      if (resC < 0) continue;
      auto gao = [](int n, int m) {
        if (n == 0) return 1;
        if (m == 0) return 0;
        return Bi(n + m - 1, m - 1);
      };
       // cout << one << " " << two << " " << three << " " << four << " " << mul(fac[one + two + three + four],
       //                                                                        gao(resC, four)) << "\n";
       ans = add(ans, mul(cao(one, two, three, four),
                         gao(resC, four)));
    }
  cout << ans << "\n";
}

Submission Info

Judge Result