# J, 26 bytes [:,@,./(0 1$~#)]`(|.@])/.] ### ungolfed [: ,@,./ (0 1 $~ #) ]`(|.@])/. ] ### explanation - ``(0 1$~#)]`(|.@])/.]`` Use Key `/.` to split the input into the even/odd groups: `(0 1$~#)` creates the group definition, by repeating 0 and 1 cyclically to the length of the input. We use the gerundial form of Key for its main verb ``]`(|.@])``, which applies the identity to the first group and reverses the second group: `(|.@])`. - Now that we have the two groups, the odd one reversed, we just zip them together and flatten: `,@,./` [Try it online!](https://tio.run/##PYpJC4JAGEDv/oqvBVQYRrsKgvuejfsSQpBKxMCAXqu/boLg4V3ee@9lnEFVQIaV5a4gDWFJkOFy/p3E7iF8sNaJEu4WkTti4EdV4QHBV4Fx5rjh@WIwAu8NlDIEFZtof@AhNjB4PZ1YBYhROhz2kWyR7EI3TMt2XM8Pwuga30iSZnlRVnXTbqfemrVdurmfhmTtURJkXuFUVmMsfw "J – Try It Online")