Submission #19061

#	Time	Username	Problem	Language	Result	Execution time	Memory
19061		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