找出斐波那契数

1、递归方法  时间复杂度（O(2ⁿ)）

public static long fibonacci(long index) {

        if (index == 0) {
            return 0;
        }
        else if (index == 1) {
            return 1;
        }
        else {
            return fibonacci(index - 1) + fibonacci(index - 2);
        }
    }

2、通过对数列中的前两个数相加   时间复杂度（O(n)）

public static long fib(long index) {

        int fib0 = 0;
        int fib1 = 1;
        int fib2 = 1;
        if (index == 0) {
            return fib0;
        }
        else if (index == 1) {
            return fib1;
        }
        else if (index == 2) {
            return fib2;
        }
        else {
            for (int i = 3; i <= index; i++) {
                fib0 = fib1;
                fib1 = fib2;
                fib2 = fib0 + fib1;
            }
            return fib2;
        }
    }