一些python算法题

# 今日头条测试开发面试编程题：123456789隔三个字符反转成321654987


def reverse_by_3(s):
    s1 = ''
    while True:

        first = s[:3]
        s1 += first[::-1]
        s = s[3:]
        if len(s)<3:
            s1 += s
            break


    return s1