public class Solution
extends Object
17 - Letter Combinations of a Phone Number\.
Medium
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 digit 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']`.