Letter Combinations of a Phone Number

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'].

Test case 1

Input

digits = "23"

Output

["ad","ae","af","bd","be","bf","cd","ce","cf"]