实现一个trim()函数，去除字符串首尾的空格

def trim(s):
    if s[:1] != ' ' and s[-1:] != ' ':
        return s
    elif s[:1] == ' ':
        return trim(s[1:])
    else :
        return trim(s[:-1])

print(trim("  hello  "))
print("   hello   ")