求取斐波拉契數列第N位的值

斐波拉契數列：每一位的值等於他前兩位數字的和。前兩位固定

0、1、1、2、3、5、8、13...

public class Fib {
    public static void main(String[] args) {
        System.out.println(calculate1(10));
        System.out.println(calculate2(10));
        System.out.println(calculate3(10));
    }

    //暴力遞迴
    public static int calculate1(int num){
        if (num==0){
            return 0;
        }
        if (num==1){
            return 1;
        }
        return calculate1(num-1)+calculate1(num-2);
    }

    //改進
    public static int calculate2(int num){
        int[] arr = new int[num+1];

        return recurse(arr,num);
    }
    private static int recurse(int arr[],int num){
        if (num==0){
            return 0;
        }
        if (num==1){
            return 1;
        }
        if(arr[num] != 0){
            return arr[num];
        }
        arr[num] = recurse(arr,num-1)+recurse(arr,num-2);
        return arr[num];
    }
    //雙指標
    public static int calculate3(int num){
        int low=0,high=1;
        if (num==0){
            return 0;
        }
        if (num==1){
            return 1;
        }
        for (int i=1;i<num;i++){
            int sum = low+high;
            low = high;
            high = sum;
        }
        return high;
    }

}