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.

Example 1:

• Input: digits = “23”
• Output: [“ad”,”ae”,”af”,”bd”,”be”,”bf”,”cd”,”ce”,”cf”]

Example 2:

• Input: digits = “”
• Output: []

Example 3:

• Input: digits = “2”
• Output: [“a”,”b”,”c”]

Constraints:

• 0 <= digits.length <= 4
• digits[i] is a digit in the range [‘2’, ‘9’].