Letter Combinations of a Phone Number

On an old phone keypad, each key from 2 to 9 carries a few letters. To type a word you press the key that holds each letter, so one string of key presses can stand for many different words.

In full: 2 → abc, 3 → def, 4 → ghi, 5 → jkl, 6 → mno, 7 → pqrs, 8 → tuv, 9 → wxyz.

You get the key presses as a string digits. Return every letter string they could spell: one letter from each pressed key, in the order the keys were pressed. You may return the strings in any order.

Constraints

• 1 <= digits.length <= 4
• Each digits[i] is one of '2' to '9'.