The standard 12-key telephone keypad looks like the following:

To write text using the keypad you have to press the number keys. For example, to write `X`

press the key 9 twice. Given a sequence of numbers, return all possible combinations of words formed from mobile keypad which have some digits associated with each key.

When the solution is called with `arr = [2, 3, 4]`

the expectected list of permutations is the following:

```
ADG BDG CDG AEG BEG CEG AFG BFG CFG ADH BDH CDH AEH BEH CEH AFH BFH CFH ADI BDI CDI AEI BEI CEI AFI BFI CFI
```

Examples:

- ADG is, press 2 once, press 3 once, press 4 once.
- BDG is, press 2 twice, press 3 once, press 4 once.

Contributed by Murat Sütunç

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