Letter Combinations of a Phone Number

You get a string digits that contains digits from 2 to 9. On a phone keypad, each of these digits has a group of letters.

The digits 2 to 5 have these letters: 2 has abc, 3 has def, 4 has ghi and 5 has jkl. The digits 6 to 9 have these letters: 6 has mno, 7 has pqrs, 8 has tuv and 9 has wxyz.

Return all the strings that you can make when you choose one letter for each digit. Keep the letters in the same order as their digits.

You can return the strings in any order. If digits is empty, return an empty list.