HDU 1568（Fibonacci）

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

int main()
{
    int f[21] = { 0,1 }; //前20個Fibonacci數
    for (int i = 2; i < 21; i++)
    {
        f[i] = f[i - 1] + f[i - 2];
    }
    int n;
    double s = (sqrt(5.0) + 1.0) / 2.0;
    while (cin >> n)
    {
        if (n < 21)
        {
            cout << f[n] << endl;
            continue;
        }
        double ans = -0.5 * log(5.0) / log(10.0) + ((double)n) * log(s) / log(10.0);
        ans -= floor(ans);
        ans = pow(10.0, ans);
        while (ans < 1000)
        {
            ans *= 10;
        }
        cout << (int)ans << endl;
    }
    return 0;
}