if len(s) == 0:    #空字符串
    return "" 
n = 0              #全是空格
for i in s:        
    if i != " ":
        break
    n += 1;
if n == len(s):
    return ""
while True:        #掐头
    if s[0] == " ":
        s = s[1:]
    else:
        break
while True:        #去尾
    if s[-1] == " ":
        s = s[:-1]
    else:
        break 
return s