Submission #1150386

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Source Code Expand

#include <set>
#include <map>
#include <cmath>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <vector>
#include <bitset>
#include <string>
#include <cstdio>
#include <cassert>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <algorithm>
#define mk make_pair
#define pb push_back
#define fi first
#define se second
#define REP(i, x, y) for(int i = (int)x; i <= (int)y; i ++)
#define FOR(i, x, y) for(int i = (int)x; i <  (int)y; i ++)
#define PER(i, x, y) for(int i = (int)x; i >= (int)y; i --)
#define trace(x) cerr << #x << " " << x << endl;
#define dprintf(...) fprintf(stderr, __VA__ARGS__)
#define dln()        fprintf(stderr, "\n")
using namespace std;
typedef long long LL;
typedef long double db;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef vector<PII> VPI;
const	int N = 100005;
const	int P = 1e9 + 7;
const	int inf = 1e9;
const	LL Inf = 1e15;

inline int IN(){
	char ch = getchar(); int x = 0, f = 0;
	while(ch < '0' || ch > '9') ch = getchar(), f = (ch == '-');
	while(ch >= '0' && ch <= '9'){
		x = (x << 1) + (x << 3) + ch - 48;
		ch = getchar();
	}
	return f ? (-x) : x;
}

inline int Pow(int x, int y, int p){
	int an = 1;
	for(; y; y >>= 1, x = (LL)x * x % p) if(y & 1) an = (LL)an * x % p;
	return an;
}

template<typename T> inline void chkmin(T &a, const T &b) {if(a > b) a = b;}
template<typename T> inline void chkmax(T &a, const T &b) {if(a < b) a = b;}

int n, A, B, C;
int fac[10005], ifac[10005];

int Comb(int x, int y){
	if(y < 0 || x < y) return 0;
	return (LL)fac[x] * ifac[y] % P * ifac[x - y] % P;
}

int AA = 0;
int ask(int w){
	//\sum{a[i]} = B / 2
	//\sum{i * a[i]} = w
	//sum of \prod{a[i]!}
	if(B == 0) return w == 0;
	return (LL)AA * Comb(B / 2 + w - 1, B / 2 - 1) % P;	
}

int main(){
	cin >> n >> A >> B >> C;
	if(B & 1){
		puts("0");
		return 0;
	}
	ifac[0] = 1;
	REP(i, 1, 10000) ifac[i] = (LL)ifac[i - 1] * Pow(i, P - 2, P) % P;
	REP(i, 0, 10000) fac[i] = Pow(ifac[i], P - 2, P);
	AA = ifac[B / 2];
	int ans = 0;
	REP(a, 0, A) for(int b = 0; 3 * b <= C; b ++){
		int ww = C - A + a - 3 * b;
		if(ww < 0) continue;
		int cf = (LL)fac[A + b + B / 2] * ifac[a] % P * ifac[A - a] % P * ifac[b] % P;
		cf = (LL)cf * ask(ww) % P;
		ans = (ans + cf) % P;
	}
	printf("%d\n", ans);
	return 0;
}

Submission Info

Judge Result