算法（递归）

1：阶乘

def factorial(n):
    if n == 1:
        return 1
    else:
        return n * factorial(n-1)

2：幂

def power(x, n):
    if n == 0:
        return 1
    else:
        return x * power(x, n-1)

3：斐波那契数列

def Fibonacci(num):
    if num <= 1:
        return 1
    else:
        return Fibonacci(num -1) + Fibonacci(num - 2)