power(x,n)函数是计算x的n次幂的函数，power(x,n)函数运用了分治算法的思想，调用power(3,3)函数的计算结果是：27

def power(x,n):

    if n == 2:

        return x

    if n%2 == 0:

        return power(x,n//2) * power(x,n//2)

    else:

        return power(x,(n+1)//2) * power(x,(n-1)//2)