Submission #19949

#	Time	Username	Problem	Language	Result	Execution time	Memory
19949		yukariko	괄호 (kriii4_R)	C++14	0 / 100	6 ms	32968 KiB

R

#include <bits/stdc++.h>

using namespace std;

const int MOD = 1000000007;

int N, M;

long long cache[1000001];

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 m_cache[1000001];
long long mod_div(long long a, long long b)
{
	if(m_cache[a])
		return m_cache[a];
	return m_cache[a] = (a * mod_exp(b, MOD-2, MOD)) % MOD;
}

long long solve(int pos, int left)
{
	if(left < 0)
		return 0;
	if(pos == N)
		return 1;

	long long ans = (solve(pos + 1, left - 1));
	ans = (ans + solve(pos + 1, left + 1) * M) % MOD;
	return ans;
}

long long pow2[1000001];
long long nCm[1000001];

int main()
{
	scanf("%d%d", &N, &M);

	pow2[1] = 1;
	for(int i=2; i <= N; i++)
		pow2[i]= (pow2[i-1] * 2) % MOD;

	nCm[0] = 1;
	nCm[1] = 1;
	for(int i=2; i <= N/2; i++)
	{
		int n = i-1;
		nCm[i] = (nCm[i-1] * (n * 2 + 1)) % MOD;
		nCm[i] = (nCm[i] * (n * 2 + 2)) % MOD;
		nCm[i] = mod_div(nCm[i], (n + 1) * (n + 2) % MOD );
//		printf("%lld ", nCm[i]);
	}

	cache[0] = 1;
	cache[1] = M;
	cache[2] = M * (M+1);
	for(int i=3; i <= N; i+=2)
	{
		//printf("%lld ", nCm[i/2] * pow2[(i+1)/2]);
		cache[i] = ((cache[i-1] * (M+1) % MOD) - (pow2[(i+1)/2] * nCm[i/2] % MOD) + MOD) % MOD;
		cache[i+1] = cache[i] * (M+1) % MOD;
		//printf("%lld %lld ", cache[i], cache[i+1]);
	}

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

• Subtask 1: 0 / 8
• Subtask 2: 0 / 92