Post History

# [Jelly], 2 bytes

    ŒṪ

[Try it online!][TIO-kkeoqel6]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-kkeoqel6]: https://tio.run/##y0rNyan8///opIc7V/0/3P6oac2jhpn//0dHG@oY6BiCoUFsrA6UD2TAaAOIDAA "Jelly – Try It Online"

1-indexed, but the footer converts to 0-indexed.

Less built-in:

# [Jelly], 3 bytes

    œẹ1

[Try it online!][TIO-kkeoqods]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-kkeoqods]: https://tio.run/##y0rNyan8///o5Ie7dhr@P9z@qGnNo4aZ//9HRxvqGOgYgqFBbKwOlA9kwGgDiAwA "Jelly – Try It Online"

```
œẹ     All multidimensional indices of
  1    one.
```

Less built-in:

# [Jelly], 7 bytes

    J,Ɱ"T€Ẏ

[Try it online!][TIO-kkeorhym]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-kkeorhym]: https://tio.run/##y0rNyan8/99L59HGdUohj5rWPNzV9/9wO5DxqGHm///R0YY6BjqGYGgQG6sD5QMZMNoAIgMA "Jelly – Try It Online"

```
 ,Ɱ        Pair each
    T      truthy index
     €     from all rows
J  "       with the indices of the rows.
      Ẏ    Flatten rows.
```

The absolute least built-in:

# [Jelly], 8 bytes

    Jx),€"JẎ

[Try it online!][TIO-kkep05j5]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-kkep05j5]: https://tio.run/##y0rNyan8/9@rQlPnUdMaJa@Hu/r@H24HMh81zAz9/z862lDHQMcQDA1iY3WgfCADRhtAZAA "Jelly – Try It Online"

Outputs each index backwards.

```
Jx          Repeat each index by the corresponding element
  )         for each row.
   ,€"J     Pair each remaining index with its row index.
       Ẏ    Flatten rows.
```