Submission #19205

#	Time	Username	Problem	Language	Result	Execution time	Memory
19205		kriii	Ω (kriii4_P3)	C++14	100 / 100	0 ms	1084 KiB

P3

#include <stdio.h>

const long long mod = 1000000007;

long long fpow(long long a, long long p)
{
	a = (a % mod + mod) % mod;
	p = (p % (mod - 1) + mod - 1) % (mod - 1);
	long long r = 1;
	while (p){
		if (p & 1) r = r * a % mod;
		a = a * a % mod;
		p >>= 1;
	}
	return r;
}

int main()
{
	int P,Q,N,K; scanf ("%d %d %d %d",&P,&Q,&N,&K);

	if (K == 0) puts("0");
	else if (K == N) puts("1");
	else if (Q == 0) puts("1");
	else if (Q == P) puts("0");
	else{
		long long v[101];
		long long pr = Q * fpow(P,-1) % mod, rv = fpow(1-pr,-1);
		v[0] = 0; v[1] = 1;
		for (int i=2;i<=N;i++) v[i] = ((v[i-1] - pr * v[i-2]) % mod * rv % mod + mod) % mod;
		printf ("%lld\n",v[K]*fpow(v[N],-1)%mod);
	}

	return 0;
}

• Subtask 1: 100 / 100