实现阶乘

#
#
# @param x double浮点型
# @param n double浮点型
# @return double浮点型
#
class Solution:
    def pow(self , x , n ):
        # write code here
        if n==0:
            return 1
        if n==1:
            return x**n
        if n%2==0:
            if n<0:
                return 1/self.pow(x*x,-n/2)
            return self.pow(x*x,n/2)
        else:
            if n<0:
                return 1/(x*self.pow(x*x,-n//2))
            return x*self.pow(x*x,n//2)