下面的代码elif和else语句里为什么一定要加return 输出才正确，为什么不能直接递归调用函数
def trim(s):
    if s[:1] != " " and s[-1:] != " ":
        return s
    elif s[:1] == " ":
        return trim(s[1:])
    else:
        return trim(s[:-1])

if __name__ == "__main__":
    s = trim("  hello")
    print s