def trim(s):    n = len(s)    head_flag = True    foot_flag = True    while n >= 0:        n -= 1        if s[0:1] == ' ':            s = s[1:]        else:            head_flag = False        if s[-1:] == ' ':            s = s[:-1]        else:            foot_flag = False        if head_flag and foot_flag:            return s