Code Golf
string sorting add tag
Given a string as an input (which can be any acceptable/convenient format in your language), implement pendulum encoding. The test cases are split into individual items (which aren't quoted) for a visually appealing explanation.
## How do I do that?
The current **iteration index** starts at `0`.

* If the iteration index is even, append the current item onto the output string.
* If the iteration index is odd, prepend the current item onto the output string.

## An example
The input is [a b c d e f g].
Note that the letters a-g are individual one-character strings, to prevent confusion from the iteration index.
N: the iteration index

N:0 Out:      [a]
N:1 Out:    [b a]
N:2 Out:    [b a c]
N:3 Out:  [d b a c]
N:4 Out:  [d b a c e]
N:5 Out:[f d b a c e]
N:6 Out:[f d b a c e g]
The output should be `[f d b a c e g]`.
# Another example
The input is [u d l n u e m p].

N:0 Out:        [u]
N:1 Out:      [d u]
N:2 Out:      [d u l]
N:3 Out:    [n d u l]
N:4 Out:    [n d u l u]
N:5 Out:  [e n d u l u]
N:6 Out:  [e n d u l u m]
N:7 Out:[p e n d u l u m]
## Test cases
[Here]('s a sample program doing this encoding.

Take note that the characters in the string aren't always unique.
Your output *has* to be flattened.

[a,b,c,d,e,f,g]   -> [f,d,b,a,c,e,g]
[]                -> []
[a]               -> [a]
[a,b,c,d]         -> [d,b,a,c]
[a,b]             -> [b,a]
[a,b,d]           -> [b,a,d]
[a,b,a,c,b,c]     -> [c,c,b,a,a,b]
[a,a,b,b,c,c]     -> [c,b,a,a,b,c]
[u,d,l,n,u,e,m,p] -> [p,e,n,d,u,l,u,m]
Top Answer APL (Dyalog Extended)
# [APL (Dyalog Extended)], 11 [bytes]( "When can APL characters be counted as 1 byte each?") ([SBCS]( ".dyalog files using a single byte character set"))


[Try it online!][TIO-k88l7mbf] (Empty case works offline, but TIO hasn't been updated.)

`-\` alternating sum

`` `∘`` of

`⍳` **ɩ**ndices

`∘` of

`≢` the length

`⍋` graded (indices that would sort)

`⍛` before

`⊇` being used to select from

`⊢` the argument

[APL (Dyalog Extended)]:
[TIO-k88l7mbf]: "APL (Dyalog Extended) – Try It Online"
Answer #2 Python 3
# [Python 3] REPL, 25 bytes


[Try it online!][TIO-khqsf6k8]

## Explanation
    _[           ::  ]          substring
                   -2            of every other item, backwards, from
        len(_)                    the length
              %2                  mapped even => 0
                                          odd => 1
      ~(        )                 mapped 0 => -1 (last element)
                                         1 => -2 (second-last)
                      +        joined with
                       _[:: ]    substring
                           2      of every other item

[Python 3]:
[TIO-khqsf6k8]: "Python 3 – Try It Online"
Answer #3
# [Jelly], 7 bytes


[Try it online!][TIO-khpc4b88]

[TIO-khpc4b88]: "Jelly – Try It Online"

## Explanation

    ŒœṚ;¥@/   Main monadic link
    Œœ        Split into odd and even indices
          /   Fold over
         @      Reverse arguments
        ¥         Dyad (
      Ṛ             Reverse [left argument]
       ;            Join [with right argument]
        ¥         )

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