def trim(s):
n=len(s)
while n >= 0:
if s[:1] == ' ':
s=s[1:]
if s[-1:] == ' ':
s=s[:-1]
n= n - 1
return s
测试trim(' ')程序报错字符串索引越界，把第二个if修改为elif
抱歉看错了，代码是可行的QAQ
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def trim(s):
n=len(s)
while n >= 0:
if s[:1] == ' ':
s=s[1:]
if s[-1:] == ' ':
s=s[:-1]
n= n - 1
return s