hdu 1568 Fibonacci

水题，注意一点就是前20位不能用公式，后面才可以用……

#include <iostream>
#include <cmath>
using namespace std;
int ans[22]={0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765};
int fun(long n)
{
double a;
int b;
a=-0.5*log10(5.0)+n*log10((sqrt(5.0)+1)/2);
b=pow(10.0,a-(int)a+3);
return b;
}
int main()
{
int n,sum,num,t=0;
while(scanf("%d",&n)!=EOF)
{
if(n<=20)
cout<<ans[n]<<endl;
else cout<<fun(n)<<endl;
}
return 0;
}