hdu 1410

Ref : http://www.clanfei.com/2012/04/772.html

可推出公式：

ans=sigma[C(N1-1,i)*0.5^(N1+i)]　　(0<=i<=N2-1)

注意double相乘会掉精度导致WA，所以要用黑科技：

log(a*b)=log(a)+log(b)

10^(log(a))=a

这样就把乘法转成加法辣~

#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;

double ans,tmp;
int N1,N2,HP1,HP2,AP1,AP2;

int main()
{
    while(cin>>HP1>>HP2>>AP1>>AP2)
    {
        N2=HP1/AP2;        if(HP1%AP2!=0)    N2++;
        N1=HP2/AP1;        if(HP2%AP1!=0)    N1++;
        
        ans=pow(0.5,N1);
        //tmp=1.00;
        tmp=0;
        for (int i=1;i<=N2-1;i++)
        {
            //tmp=tmp*(N1-i)/i;
            //ans+=tmp*pow(0.5,N1+i);
            tmp+=log10(N1-1.0+i)-log10(i*1.0);
            ans+=pow(10,tmp+(N1+i)*log10(0.5));
        }
        ans=ans*100;
        printf("%.4f
",ans);
    }
}