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 <= 4digits[i]is a digit in the range['2', '9'].