hdu-Fibonacci

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <map>
#include <vector> 
#include <cmath>
using namespace std;
int main()  
{  
    int f[22]={0,1,1};  
    for (int i = 3 ; i <= 20 ; i++)  
        f[i] = f[i-1] + f[i-2];  
    double n;  
    double a1 = log10(1.0 / sqrt(5));  
    double a2 = log10((1 + sqrt(5)) / 2);  
    while (cin>>n)  
    {  
        if (n <= 20)  
        {  
            cout<<f[(int)n]<<endl;  
            continue;  
        }  
        double ans = a1 + n * a2;  
        ans -= floor(ans);  
        ans = pow (10,ans);  
        ans = (int)(ans * 1000);  
        cout<<ans<<endl;  
    }  
    return 0;  
}