#循环:

def trim(s):
    while s[:1] == ' ':
            s = s[1:]
    while s[-1:] == ' ':
            s = s[:-1]
    return s

#递归:

def trim(s):
    if s[:1] == ' ':
        s = trim(s[1: ])
    if s[-1:] == ' ':
        s = trim(s[:-1])
    return s