Phone Combinations

Problem

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

2 - "abc"
3 - "def"
4 - "ghi"
5 - "jkl"
6 - "mno"
7 - "pqrs"
8 - "tuv"
9 - "wxyz"

Note: Although the problem statement might suggest otherwise, ensure your solution can handle inputs with multiple digits, generating all combinations recursively or iteratively.

Examples

letterCombinations("2");
// returns ["a","b","c"]
// '2' could correspond to 'a', 'b', or 'c'.

letterCombinations("23");
// returns ["ad","ae","af","bd","be","bf","cd","ce","cf"]
// '2' corresponds to 'a', 'b', or 'c';
// '3' corresponds to 'd', 'e', or 'f'
// The combinations are the product of these sets.