Submission #1000915

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Source Code Expand

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<int, int> PII;

#define fi first
#define se second
#define mp make_pair
#define pb push_back

#define P 1000000007

#define NN 100010

ll F[NN], IF[NN];
void gcd(int a,int b,int &x,int &y) {
  if(b==0){x=1,y=0;return;}
  gcd(b,a%b,y,x),y-=a/b*x;
}
int inv(int n) {
  int x,y;gcd(n,P,x,y);
  if(x<0)x+=P;return x;
}

ll cc(int n, int m) {
	//printf ("%d %d\n", n, m);
	if (m < 0) return 0;
	if (m > n) return 0;
	return (ll) F[n] * IF[m] %P * IF[n-m]%P;
}

ll ff(int m, int k) {
	if (k == 0) {
		if (m == 0) return 1;
		return 0;
	}
	return cc(m+k-1, k-1);
}

int main () {
	F[0]=1;for(int i=1;i<NN;i++)F[i]=(ll)F[i-1]*i%P;
	IF[NN-1]=inv(F[NN-1]);for(int i=NN-1;i;i--)IF[i-1]=(ll)IF[i]*i%P;
	int N, A, B, C;
	cin >> N >> A >> B >> C;
	if (B&1) {
		puts ("0");
		return 0;
	}
	ll S = 0;
	for (int j = 0; j*3 <= C; j++) {
		C -= 3*j;
		for (int i = 0; i <= C; i ++) {
			if (A-(C-i) < 0) continue;
			ll T = ff(i, B/2);
			// C-i : 31
			T = (ll)T*ff(C-i, B/2+1)%P;
			// A-(C-i) : 1
			T = (ll)T*ff(A-(C-i), B/2 + (C-i) + 1)%P;
			
			T = (ll)T*ff(j, B/2 + A + 1)%P;
			(S += T) %= P;
		}
		C += 3*j;
	}
	cout << S << endl;
	return 0;
}

Submission Info

Judge Result