Submission #56620

#	Time	Username	Problem	Language	Result	Execution time	Memory
56620		ksun48	Tents (JOI18_tents)	C++14	48 / 100	2064 ms	5432 KiB

tents

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;

const LL MOD = 1000000007;

LL powmod(LL a, LL n){
	if(n == 0) return 1;
	if(n % 2) return (a * powmod(a, n-1)) % MOD;
	LL c = powmod(a, n/2);
	return (c * c) % MOD;
}

LL inv(LL a){
	return powmod(a, MOD-2);
}
LL fact[11000];
LL invfact[11000];

int main(){
	LL a, b;
	cin >> a >> b;
	fact[0] = invfact[0] = 1;
	for(LL i = 1; i < 11000; i++){
		fact[i] = (i * fact[i-1]) % MOD;
		invfact[i] = inv(fact[i]);
	}

	LL smth[a+1][b+1];
	for(int i = 0; i <= a; i++){
		for(int j = 0; j <= b; j++){
			if((i + j) % 3 != (a + b) % 3) continue;
			LL cur = 0;
			for(int k = 0; k <= i && k <= j; k++){
				LL z = powmod(4,k);
				z = (z * invfact[k]) % MOD;
				z = (z * invfact[i-k]) % MOD;
				z = (z * invfact[j-k]) % MOD;
				cur = (cur + z) % MOD;
			}
			smth[i][j] = cur;
		}
	}

	LL ans = 0;
	for(LL c = 0; c <= a && c*2 <= b; c++){
		for(LL d = 0; c + 2*d <= a && c*2 + d <= b; d++){
			LL cur = (fact[a] * fact[b]) % MOD;
			cur = (cur * inv(powmod(2,c))) % MOD;
			cur = (cur * inv(powmod(2,d))) % MOD;
			cur = (cur * invfact[c]) % MOD;
			cur = (cur * invfact[d]) % MOD;
			cur = (cur * smth[a - c - 2*d][b - c*2 - d]) % MOD;
			ans = (ans + cur) % MOD;
		}
	}
	ans--;
	ans %= MOD;
	if(ans < 0) ans += MOD;
	cout << ans << '\n';
}

• Subtask 1: 48 / 48
• Subtask 2: 0 / 52