#方法一def trim(s):    if not isinstance(s,str):        raise TypeError('请输入字符串。')    if s == '':        return ''    m = 0    n = len(s) - 1    while m <= len(s)-1 and s[m] == ' ':        m += 1    while n >= 0 and s[n] == ' ':        n -= 1    if m >= n :        return ''    else:        return s[m:(n + 1)]''''''#方法二def trim(s):    while s[:1] == ' ':        s = s[1:]    while s[-1:] == ' ':        s = s[:-1]    return s'''''''#方法三def trim(s):    if s[:1] == ' ':        return trim(s[1:])    elif s[-1:] == ' ':        return trim(s[:-1])    else:        return s