1568.Fibonacci

题目链接
http://acm.hdu.edu.cn/showproblem.php?pid=1568

思路
参考了大神的对数解法：
http://www.cnblogs.com/Yu2012/archive/2011/10/09/2199156.html
代码：

#include<iostream>
#include<math.h>

using namespace std;

int Fib(int n)
{
    if (n == 0)
        return 0;
    else
        return n < 2 ? 1 : (Fib(n - 1) + Fib(n - 2));
}

int main()
{
    int n;
    double f = (sqrt(5.0) + 1) / 2;
    double a;
    while (cin >> n)
    {
        if (n < 21)
        {
            cout << Fib(n) << endl;
        }
        else {
            a = -0.5*log10(5.0) + n*log10(f);
            a = a - int(a);
            a = pow(10.0, a);
            while (a < 1000)
            {
                a *= 10;
            }
            cout << (int)a << endl;
        }
    }

    return 0;
}