O(1)斐波那契求逆

O(1)通项公式解法：

import java.util.*;
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        double x = sc.nextDouble();
        double a = Math.sqrt(5.0);
        double temp1 = (1.0 + a)/2.0;
        double temp2 = (1.0 - a)/2.0;
        double r = Math.log(x*a)/Math.log(temp1);//利用通项公式求这是第几项，忽略较小幂
        int res = (int)Math.round(r);
        int k = (int)(1.0/a*(Math.pow(temp1, res) - Math.pow(temp2, res)));
        System.out.println((int)Math.abs(x - k));
    }
}