Letter Combinations of Digits from the Phone Pad

The function is given a string containing digits from 2 to 9. Return a set of all possible letter combinations that could represent the digit-string.

Digits to Letters Mapping

d = { "2": "abc", "3": "def", "4": "ghi", "5": "jkl", "6": "mno", "7": "pqrs", "8": "tuv", "9": "wxyz" }

Examples

letters_combinations("23") ➞ { "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf" }

letters_combinations("") ➞ set()

letters_combinations("2") ➞ { "a", "b", "c" }

Notes

N/A