Submission #19843

#TimeUsernameProblemLanguageResultExecution timeMemory
19843yukariko괄호 (kriii4_R)C++14
8 / 100
1536 ms19300 KiB
#include <bits/stdc++.h>

using namespace std;

const int MOD = 1000000007;

int N, M;

long long cache[2000001];
char res[2000001];

long long mod_exp(long long a,long long b,long long c)
{
	a%=c;
	if(b==0)return 1;
	if(b==1)return a;
	if(b&1) return (a*mod_exp((a*a)%c,(b-1)/2,c))%c;
	return mod_exp((a*a)%c,b/2,c);
}

long long mod_div(long long a, long long b)
{
	return (a * mod_exp(b, MOD-2, MOD)) % MOD;
}

long long solve(int pos)
{
	if(pos > 2 * N)
		return 0;
	if(pos == 2 * N)
	{
		puts(res);
		return 1;
	}

	long long& ans = cache[pos];
	//if(ans != -1)
	//	return ans;

	res[pos / 2] = '(';
	res[2 * N - (pos / 2) - 1] = ')';
	ans = (solve(pos + 2) * M) % MOD;
	res[pos / 2 + 1] = ')';
	res[2 * N - (pos / 2) - 2] = '(';
	ans = (ans + solve(pos + 4) * M) % MOD;
	return ans;
}


int main()
{
	memset(cache, -1, sizeof(cache));
	scanf("%d%d", &N, &M);

	cache[0] = M;
	cache[1] = M;
	for(int i=2; i <= N; i++)
	{
		cache[i] = (cache[i-1] * 2 + cache[i-2] * 4 * (i - 1)) % MOD;
		cache[i] = mod_div(cache[i], i+1);
	}

	printf("%lld\n", cache[N]);
	return 0;
}
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