Java、斐波那契数

package Recursive;
 
import java.math.BigInteger;
import java.util.Scanner;
 
public class Exercise18_02 {
 
    public static void main(String[] args) {
        System.out.print("Enter an index for fibonacci number: ");
        int index = new Scanner(System.in).nextInt();
 
        if (index < 0)
            System.out.println("Invalid index!");
        else 
            System.out.println("The fibonacci number at index " + index + " is " + fib(BigInteger.valueOf(index)));
    }
 
    /** 返回斐波那契数 */
    public static BigInteger fib(BigInteger index) {
        BigInteger f0 = BigInteger.ZERO;    //第一个数
        BigInteger f1 = BigInteger.ONE;     //第二个数
        BigInteger result = BigInteger.ZERO;    //结果
 
        if (index.equals(BigInteger.ZERO))  //如果等于0
            return f0;
        else if (index.equals(BigInteger.ONE))  //如果等于1
            return f1;
        else {  //否则
            for (BigInteger i = BigInteger.ONE; i.compareTo(index) == -1; i = i.add(BigInteger.ONE)) {
                result = f0.add(f1);
                f0 = f1;
                f1 = result;
            }
            return result;
        }
    }
}