Given a string containing digits from `2-9` inclusive, return all possible letter combinations that the number could represent. Return the answer in **any order**.
A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
- 2: abc
- 3: def
- 4: ghi
- 5: jkl
- 6: mno
- 7: pqrs
- 8: tuv
- 9: wxyz