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 <= 4digits[i] is a digit in the range ['2', '9'].假設給一個數字對應到字元的方式，如同手機按鍵一樣
2 -> a, b, c
3 -> d, e, f
...
要求寫一個演算法來從給定的一個數字組成的字串找出所可能透過上面對應方式組出的字串