作业不是越简单越好嘛= = 

#!/usr/bin/env python3

# -*- coding: utf-8 -*-

def trim(str):

if len(str)==0:

return str

elif str[0]==" " and str[-1]==" ":

# print(str[1:-1])

return trim(str[1:-1])

elif str[0]==" ":

# print(str[1:])

return trim(str[1:])

elif str[-1]==" ":

# print(str[0:-1])

return trim(str[0:-1])

else:

return str