Submission #19406

#	Time	Username	Problem	Language	Result	Execution time	Memory
19406		imsifile	Ω (kriii4_P3)	C++98	0 / 100	0 ms	1084 KiB

P3

#include<stdio.h>
#define mod 1000000007

typedef long long lld;
lld a, b, sum;

lld exp(lld a, lld b){
	if(b==0)return 1;
	lld k=exp(a, b/2);
	k=(k*k)%mod;
	if(b%2)k=(k*a)%mod;
	return k;
}

int n, k;
lld p, q, pq, mpq, dy[111], re;
double pr, d2[111];

int main(){
	int n, i;
	scanf("%lld%lld%d%d", &p, &q, &n, &k);
	if(k==0 || p==q){puts("0"); return 0;}
	if(k==n || q==0){puts("1"); return 0;}

	pr = (double)q/p;
	pq = q*exp(p,mod-2)%mod, mpq = (mod-pq)%mod;
	dy[1]=1;
	for(i=1; i<n; i++)dy[i+1] = (dy[i] + mpq*dy[i-1]%mod)*exp(1+mpq, mod-2)%mod;
	d2[1]=1;
	for(i=1; i<n; i++)d2[i+1] = (d2[i] - pr*d2[i-1])/(1-pr);
//	for(i=0; i<=n; i++)printf("%lf ", d2[i]/d2[n]);
//	puts("");
	printf("%lld", dy[k]*exp(dy[n],mod-2)%mod);
	return 0;
}

• Subtask 1: 0 / 100