斐波那契数列 算法实现

斐波那契数列, 这个数列从第3项开始，每一项都等于前两项之和。

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...

递归 ：

function fibonacci(n, first, second) {
        if(n == 1) {
            return first || 0;
        }else if(n == 2){
            return second || 1;
        } else {
            return fibonacci(n - 1, first, second) + fibonacci(n - 2, first, second);
        }
}

var fb_sum = fibonacci(10, 0, 1);
console.log(fb_sum); //34

非递归:

function fibonacci_2(n, first, second){
        if(n == 1) return first || 0;
        if(n == 2) return second || 0;

        var sum = second, temp = 0, lastSum = first;

        for(var i = 2; i < n; i++){
            temp = sum;
            sum += lastSum;
            lastSum = temp;
        }
        return sum;
}



var fb_sum = fibonacci_2(10, 0, 1);
console.log(fb_sum); // 34